Relaxing French art treasures originally pinched by gang (7)
I believe the answer is:
resting
'relaxing' is the definition.
(I know that relaxing can be written as resting)
'french art treasures originally pinched by gang' is the wordplay.
'french art' becomes 'es' ('is' in French - 'art' can mean 'is' eg 'thou art').
'originally' says to take the initial letters.
'pinched by' indicates putting letters inside (inserted letters get pinched or taken).
'gang' becomes 'ring' (synonyms).
The initial letter of 'treasures' is 't'.
'es'+'t'='est'
'est' going inside 'ring' is 'RESTING'.
(Other definitions for resting that I've seen before include "(Of actor) not working" , "Taking it easy" , "Out of work (acting)" , "Reposing" , "Actor while out of work" .)