Surplus old paintings put up (5)
I believe the answer is:
extra
'surplus' is the definition.
('extra' can be a synonym of 'surplus')
'old paintings put up' is the wordplay.
'old' becomes 'ex'.
'paintings' becomes 'art' (I've seen this before).
'put up' says the letters should be written backwards.
'art' back-to-front is 'tra'.
'ex'+'tra'='EXTRA'
(Other definitions for extra that I've seen before include "Way of scoring at cricket" , "Additional expense" , "Actor in crowd scene, no lines" , "An increased amount" , "Left over" .)