Metal surrounds old enemy (3)
I believe the answer is:
foe
'enemy' is the definition.
(synonyms)
'metal surrounds old' is the wordplay.
'metal' becomes 'fe' (Fe is the atomic symbol for iron).
'surrounds' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fe' going around 'o' is 'FOE'.
(Other definitions for foe that I've seen before include "Evil rival" , "Opponent" , "Adversary" , "Antagonist" , "opposing side" .)