Painter Foster's spilled a thinner on the ground (5)
I believe the answer is:
rarer
'thinner on the ground' is the definition.
(thin on the ground means rare)
'painter foster's spilled a' is the wordplay.
'painter' becomes 'RA' (member of the Royal Academy of Arts).
'foster' becomes 'rear' (both can mean to bring up or raise).
'spilled' is a deletion indicator.
'rear' with 'a' taken out is 'rer'.
'ra'+'rer'='RARER'
(Other definitions for rarer that I've seen before include "even less common" , "(Of meat) less cooked" , "Comparatively sparse" , "less easy to find" , "Less often seen" .)