Fake iron and gin cocktail (5)
I believe the answer is:
feign
'fake' is the definition.
(I know that fake is a more specific form of the action feign)
'iron and gin cocktail' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'and' is a charade indicator (letters next to each other).
'cocktail' is an anagram indicator.
'gin' with letters rearranged gives 'ign'.
'fe'+'ign'='FEIGN'
(Other definitions for feign that I've seen before include "Affect" , "Pretend, create false impression" , "Make a pretence of" , "outdated shape" , "Simulate, fake" .)