Mum's curry -- poetry on occasion, one assumes! (7)
I believe the answer is:
usurper
'one assumes' is the definition.
(a usurper assumes someone else's position)
'mum's curry poetry on occasion' is the wordplay.
'on occasion' means one should take alternating letters.
'mums'+'curry'+'poetry'='mumscurrypoetry'
The alternating letters of 'mumscurrypoetry' are 'USURPER'.
(Other definitions for usurper that I've seen before include "Illegal seizer of power" , "One who illegally supplants another" , "Illegal supplanter" , "One seizes power illegally" , "Pursuer of one who takes power wrongfully (7)" .)