Little green men fib during answer (6)
I believe the answer is:
aliens
'little green men' is the definition.
(I've seen this in another clue)
'fib during answer' is the wordplay.
'fib' becomes 'lie' (a fib is a lie).
'during' indicates putting letters inside.
'answer' becomes 'ans' (abbreviation for answer).
'lie' placed into 'ans' is 'ALIENS'.
(Other definitions for aliens that I've seen before include "Outer space creatures, perhaps from Mars?" , "Extra-terrestrial beings" , "Life-forms assumed to exist beyond Earth" , "Forms of life assumed to exist outside Earth" , "Beings from another planet" .)