Guys welcomed by you once turned hostile (5)
I believe the answer is:
enemy
'hostile' is the definition.
(similar in meaning)
'guys welcomed by you once turned' is the wordplay.
'guys' becomes 'men' (guy can mean a man).
'welcomed by' means one lot of letters goes inside another (letters welcomed inside).
'you' becomes 'ye' (historical version of 'you').
'once turned' shows that the letters should be reversed in order.
'men' going inside 'ye' is 'ymene'.
'ymene' backwards is 'ENEMY'.
(Other definitions for enemy that I've seen before include "other side (like 16)" , "Yemen (anag.)" , "Opposing force" , "one in opposition" , "is against us" .)