Prisoner toiled as mixing cement (11)
I believe the answer is:
consolidate
'cement' is the definition.
(I've seen this before)
'prisoner toiled as mixing' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'mixing' indicates anagramming the letters.
'toiled'+'as'='toiledas'
'toiledas' anagrammed gives 'solidate'.
'con'+'solidate'='CONSOLIDATE'
(Other definitions for consolidate that I've seen before include "Fuse" , "Unify" , "Cement" , "Make firm" , "Unite" .)