Déjeuner sur l'herbe in art theft? (10)
I believe the answer is:
picnicking
'dejeuner sur l'herbe' is the definition.
(French title of Manet's 'Luncheon on the Grass')
'art theft?' is the wordplay.
'art' becomes 'pic' (picture, type of artwork).
'theft?' becomes 'nicking' (nick can informally mean to steal).
'pic'+'nicking'='PICNICKING'
'in' acts as a link.
(Other definitions for picnicking that I've seen before include "Taking courses in countryside" , "eating meal outside" .)