Enemy has iron ring hidden (3)
I believe the answer is:
foe
'enemy' is the definition.
(synonyms)
'iron ring hidden' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'ring' becomes 'o' (letter resembling a ring).
'hidden' indicates putting letters inside.
'fe' going around 'o' is 'FOE'.
'has' is the link.
(Other definitions for foe that I've seen before include "Opponent" , "Am archaic enemy" , "Environmentalists" , "Evil rival" , "Someone opposing" .)