Men spy duo concocting alias (9)
I believe the answer is:
pseudonym
'alias' is the definition.
(pseudonym is a kind of alias)
'men spy duo concocting' is the wordplay.
'concocting' indicates anagramming the letters (I've seen 'concocted' mean this).
'men'+'spy'+'duo'='menspyduo'
'menspyduo' anagrammed gives 'PSEUDONYM'.
(Other definitions for pseudonym that I've seen before include "Goliath, for one" , "Pen name - use my pond (anag)" , "Our cryptic crossword always carries one" , "in the style of George Eliot?" , "Lewis Carroll, for one" .)