Painter stern after spilling a thinner on the ground (5)
I believe the answer is:
rarer
'thinner on the ground' is the definition.
'painter stern after spilling a' is the wordplay.
'painter' becomes 'ra'.
'stern' becomes 'rear' ('rear' can be a synonym of 'stern').
'after spilling' suggests deleting specific letters.
'rear' with 'a' removed is 'rer'.
'ra'+'rer'='RARER'
(Other definitions for rarer that I've seen before include "less easy to find" , "(Of meat) less cooked" , "steak may be wanted thus?" , "Less often seen" , "More scarce or uncommon" .)