Opponent puts iron ring inside (3)
I believe the answer is:
foe
'opponent' is the definition.
(I know that foe is a type of opponent)
'iron ring inside' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'ring' becomes 'o' (letter resembling a ring).
'inside' indicates putting letters inside.
'fe' going around 'o' is 'FOE'.
'puts' is the link.
(Other definitions for foe that I've seen before include "Evil rival" , "Adversary" , "opposing side" , "friend never!" , "Opponent" .)