Volatile mood, also initially mad (13)
I believe the answer is:
temperamental
'volatile' is the definition.
(I've seen this before)
'mood also initially mad' is the wordplay.
'mood' becomes 'temper' (temper is one's mood or state of mind).
'initially' suggests taking the first letters.
'mad' becomes 'mental' (mentally ill).
The initial letter of 'also' is 'a'.
'temper'+'a'+'mental'='TEMPERAMENTAL'
(Other definitions for temperamental that I've seen before include "Erratic" , "Given to mood changes" , "Having a volatile disposition" , "Liable to mood swings" , "requiring delicate handling" .)