Moody leader is main trouble (8)
I believe the answer is:
mischief
'trouble' is the definition.
(I've seen this before)
'moody leader is main' is the wordplay.
'leader' suggests taking the first letters.
'main' becomes 'chief' (synonyms).
The first letter of 'moody' is 'm'.
'm'+'is'+'chief'='MISCHIEF'
(Other definitions for mischief that I've seen before include "Monkey business" , "Playful misbehaviour" , "injury" , "Shenanigans" , "Black . . . . . . . . (Evelyn Waugh)" .)