Artist's medium or moderate ability, initially (7)
I believe the answer is:
tempera
'artist's medium' is the definition.
(I've seen this before)
'moderate ability initially' is the wordplay.
'moderate' becomes 'temper' ('temper' can be a synonym of 'moderate'**).
'initially' suggests taking the first letters.
The initial letter of 'ability' is 'a'.
'temper'+'a'='TEMPERA'
'or' acts as a link.
(Other definitions for tempera that I've seen before include "Means of painting - pet mare (anag)" , "Paint made using egg-yolk" , "Painting method using emulsion" , "Artist's emulsion" , "Method of drawing" .)