Enemy puts iron ring inside (3)
I believe the answer is:
foe
'enemy' is the definition.
(synonyms)
'iron ring inside' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'ring' becomes 'O' (letter resembling a ring).
'inside' is an insertion indicator.
'fe' going around 'o' is 'FOE'.
'puts' acts as a link.
(Other definitions for foe that I've seen before include "Someone opposing" , "Environmentalists" , "Am archaic enemy" , "Antagonist" , "Adversary" .)