Sped off with painting as plunder (7)
I believe the answer is:
despoil
'plunder' is the definition.
('despoil' can be a synonym of 'plunder')
'sped off with painting' is the wordplay.
'off' is an anagram indicator.
'with' means one lot of letters go next to another.
'painting' becomes 'oil' (oil painting is a kind of painting).
'sped' anagrammed gives 'desp'.
'desp'+'oil'='DESPOIL'
'as' is the link.
(Other definitions for despoil that I've seen before include "Ravage" , "Plunder (a place)" , "Plunder - I sloped (anag)" , "Plunder, devastate" , "Plunder, rob" .)