Pieces penned by you once about adversary (5)
I believe the answer is:
enemy
'adversary' is the definition.
(thesaurus)
'pieces penned by you once about' is the wordplay.
'pieces' becomes 'men' ('man' can be a synonym of 'piece').
'penned by' is an insertion indicator.
'you once' becomes 'ye' (I've seen this in other clues).
'about' says the letters should be written backwards.
'men' going within 'ye' is 'ymene'.
'ymene' back-to-front is 'ENEMY'.
(Other definitions for enemy that I've seen before include "Rival" , "Hostile opponent" , "other side (like 16)" , "Foe, adversary" , "Yemen (anag.)" .)