Restrain European dressed in thong (5)
I believe the answer is:
leash
'restrain' is the definition.
(thesaurus)
'european dressed in thong' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'dressed in' indicates putting letters inside.
'thong' becomes 'lash' ('lash' can be a synonym of 'thong').
'e' placed inside 'lash' is 'LEASH'.
(Other definitions for leash that I've seen before include "Restraint, tether" , "Spanish city famous for swordmaking" , "Line for controlling a dog" , "Rope or chain restraint" , "Bind" .)