Opponent has iron ring put in (3)
I believe the answer is:
foe
'opponent' is the definition.
(I've seen this before)
'iron ring put in' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'ring' becomes 'o' (letter resembling a ring).
'put in' is an insertion indicator.
'fe' placed around 'o' is 'FOE'.
'has' acts as a link.
(Other definitions for foe that I've seen before include "Opponent" , "Opposition" , "opposing side" , "Am archaic enemy" , "Evil rival" .)