Rest of class sit for photographer (6)
I believe the answer is:
repose
'rest' is the definition.
(I know that rest can be written as repose)
'class sit for photographer' is the wordplay.
'class' becomes 'RE' (Religious Education, school subject).
'sit for photographer' becomes 'pose' (pose when getting a photo taken).
're'+'pose'='REPOSE'
'of' is the link.
(Other definitions for repose that I've seen before include "Relaxation" , "take it easy" , "place" , "calm" , "Rest - tranquillity" .)