Humour shown over Arp's first method of painting (7)
I believe the answer is:
tempera
'method of painting' is the definition.
(type of painting)
'humour shown over arp's first' is the wordplay.
'humour' becomes 'temper' (I have seen 'Bad humour ' mean 'temper' so perhaps 'humour' could also mean 'temper').
'shown over' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'arp's first' becomes 'a' (tst letter of 'arp').
'temper'+'a'='TEMPERA'
(Other definitions for tempera that I've seen before include "vehicle" , "Method of drawing" , "Artist's emulsion" , "Type of paint" , "Painting method using emulsion" .)