Hidden by tutu, monstrous belly (3)
I believe the answer is:
tum
'belly' is the definition.
(I've seen this before)
'hidden by tutu monstrous' is the wordplay.
'hidden' indicates a hidden word.
'TUM' is hidden in the letters of 'by tutu monstrous'.
(Other definitions for tum that I've seen before include "Corporation" , "Middle" , "Belly" , "Stomach (informal)" .)