It’s used by artists in moderate amounts, initially (7)
I believe the answer is:
tempera
'it's used by artists' is the definition.
(tempera is a type of paint)
'moderate amounts initially' is the wordplay.
'moderate' becomes 'temper' (synonyms).
'amounts initially' becomes 'a' (I can't explain this - if you can you should give a lot more credence to this answer).
'temper'+'a'='TEMPERA'
'in' acts as a link.
(Other definitions for tempera that I've seen before include "A paint and a painting technique" , "Artist's emulsion" , "Means of painting - pet mare (anag)" , "Paint made using egg-yolk" , "decorative style" .)