Soften a medium (7)
I believe the answer is:
tempera
'medium' is the definition.
(I've seen this before)
'soften a' is the wordplay.
'soften' becomes 'temper' (I've seen this before).
'temper'+'a'='TEMPERA'
(Other definitions for tempera that I've seen before include "Means of painting - pet mare (anag)" , "Method of drawing" , "may be useful to artist" , "Egg-yolk and water painting medium" , "vehicle" .)