Repose in old French art for Greek matricide (7)
I believe the answer is:
orestes
'greek matricide' is the definition.
(I've seen this in another clue)
'repose in old french art' is the wordplay.
'repose' becomes 'rest' (synonyms).
'in' indicates putting letters inside.
'old' becomes 'o' (abbreviation).
'french art' becomes 'es' ('is' in French - 'art' can mean 'is' eg 'thou art').
'o'+'es'='oes'
'rest' placed inside 'oes' is 'ORESTES'.
'for' is the link.
(Other definitions for orestes that I've seen before include "One who took vengeance" , "figure in Greek tragedy" , "Brother of Electra; so terse (anag.)" , "Killer of mother" , "He killed two" .)