Boss after Penny, reptile (10)
I believe the answer is:
copperhead
'reptile' is the definition.
(copperhead is a kind of reptile)
'boss after penny' is the wordplay.
'boss' becomes 'head' (synonyms).
'after' is a charade indicator (letters next to each other).
'penny' becomes 'copper' (copper is a kind of penny).
'head' put after 'copper' is 'COPPERHEAD'.
(Other definitions for copperhead that I've seen before include "Venomous, reddish-brown American snake" , "Highly venomous US or Australian snake" , "Policeman brains snake" , "Venomous type" , "Reptile" .)