Expend great effort and time on painting (4)
I believe the answer is:
toil
'expend great effort' is the definition.
(to toil is to work hard)
'time on painting' is the wordplay.
'time' becomes 't' (abbreviation).
'on' means one lot of letters go next to another.
'painting' becomes 'oil' (oil painting is a kind of painting).
't'+'oil'='TOIL'
'and' acts as a link.
(Other definitions for toil that I've seen before include "Manual labour" , "Work incessantly" , "Work extremely hard" , "Work hard, labour" , "Work really hard" .)