Prompt to squeeze a rump (9)
I believe the answer is:
remainder
'rump' is the definition.
(thesaurus)
'prompt to squeeze a' is the wordplay.
'prompt' becomes 'reminder' (I've seen this before).
'to squeeze' is an insertion indicator (inserted letters are squeezed inside).
'reminder' placed around 'a' is 'REMAINDER'.
(Other definitions for remainder that I've seen before include "Dispose of unsold copies of book at a reduced price" , "book going cheap" , "It's left over after division" , "Surplus" , "Scrap" .)