Support PM at once with manic reforms (13)
I believe the answer is:
accompaniment
'support' is the definition.
('accompaniment' can be a synonym of 'support')
'pm at once with manic reforms' is the wordplay.
'with' means one lot of letters go next to another.
'reforms' indicates anagramming the letters.
'pm'+'at'+'once'+'manic'='pmatoncemanic'
'pmatoncemanic' with letters rearranged gives 'ACCOMPANIMENT'.
(Other definitions for accompaniment that I've seen before include "Complement" , "Backing" , "Musical support?" , "Supporting musical part" , "piece of music to sing to?" .)