Checked with my card to make sure the wording was right, seems to be:

FLANKING FIRE: When Marine makes a ranged combat attack targeting an opposing character that was hit by another friendly character this turn, modify the opposing character's defense value by -1.

The last line states "modify the opposing character's defense value by -1" but it does not specify that it is "for that attack." So my question is, if the marine makes a shot after the target was hit by a friendly, if he misses, does the target keep the minus one modifier until the start of your next turn (or until damaged) kind of like perplex?

I would not ask if it said modify it by -1 "for that attack" but it just says modify.

It states "when Marine makes a range combat attack". Only then, if he is attacking a fig that was hit by another friendly that turn, does the def get modified by -1

I know, when he attacks it is modified, but it does not state "for that attack" and by normal modifications the -1 would therefore be removed upon taking a clix or healing a clix (like perplex) however this does not state for the attack meaning if he misses, and does not deal damage, the -1 should still be there because he did fulfill the requirements and it did not state it went away otherwise.

It doesn't give a duration, so there is no duration other than the condition being met. It has to give a specific duration for the game effect to last longer than the condition. In this case, the condition is "when Marine makes a ranged combat attack". When Marine is not making a ranged combat attack, the effect doesn't exist.

It doesn't have to state a "for the attack".

If a modifier doesn't specify a duration, it is in effect only while the thing that triggered it is occuring. Rule book, page 5:

MODIFIERS
Modifiers increase or decrease a combat
value by a specific amount and are cumulative.
Modifiers apply to a character for as long as that
character meets the modifier’s requirements,
and are added up when needed to determine
a combat value. Modifiers use the word
“modify” or “modifies”. Modifiers never set
combat values to specific numbers or halves;
they involve adding (+) or subtracting (-).

The attack being made by the Marine is the requirement in this instance. Since the attack by the Marine is what triggers this modifier, the modifier ends once the attack has resolved.

Thanks for the clearup, was thinking I could use a bunch of those guys as work-around negative perplexers.