Crazy being irritable after losing paint (6)
I believe the answer is:
mental
'crazy' is the definition.
(mentally ill)
'irritable after losing paint' is the wordplay.
'irritable' becomes 'temperamental' (temperamental can mean irritable or emotional).
'after losing' is a deletion indicator.
'paint' becomes 'tempera' (type of paint).
'temperamental' with 'tempera' removed is 'MENTAL'.
'being' is the link.
(Other definitions for mental that I've seen before include "Light allowing vehicles to turn while ahead is red" , "Involving the mind" , "Pertaining to the mind" , "Crazy" , "Nuts" .)