Rival holding back Greek fashion (5)
I believe the answer is:
forge
'fashion' is the definition.
(I know that fashion can be written as forge)
'rival holding back greek' is the wordplay.
'rival' becomes 'foe' (foe is a kind of rival).
'holding' is an insertion indicator.
'back' is a reversal indicator.
'greek' becomes 'gr' (abbreviation for Greek).
'gr' backwards is 'rg'.
'foe' enclosing 'rg' is 'FORGE'.
(Other definitions for forge that I've seen before include "Fabricate" , "Smith's workshop" , "Ironworks" , "1 across" , "Counterfeit - smithy" .)