Artistic technique mostly creating mood (6)
I believe the answer is:
temper
'mood' is the definition.
(temper is one's mood or state of mind)
'artistic technique mostly' is the wordplay.
'artistic technique' becomes 'tempera' (painting technique).
'mostly' means to remove the last letter.
'tempera' with its final letter taken away is 'TEMPER'.
'creating' is the link.
(Other definitions for temper that I've seen before include "Harden with heat and water" , "Tendency to become angry easily" , "Harden metal by heating and cooling" , "Make harder" , "Tantrum" .)