Six-footer traps extremely gullible spy (5)
I believe the answer is:
'spy' is the definition.
(I've seen this before)
'six-footer traps extremely gullible' is the wordplay.
'six footer' becomes 'ant' (an ant has six feet).
'traps' means one lot of letters goes inside another.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'gullible' with its middle removed is 'ge'.
'ant' going around 'ge' is 'AGENT'.
(Other definitions for agent that I've seen before include "Negotiator", "Operative", "Author", "Factor, spy", "He or she acts for actors".)