Have a fit prisoner, very large, cutting exercise (8)
I believe the answer is:
convulse
'have a fit' is the definition.
(have convulsions)
'prisoner very large cutting exercise' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'very' becomes 'v' (abbreviation).
'large' becomes 'L' (eg in clothes sizes).
'cutting' is an insertion indicator (some letters cut their way into a word).
'exercise' becomes 'use' (synonyms).
'l' put into 'use' is 'ulse'.
'con'+'v'+'ulse'='CONVULSE'
(Other definitions for convulse that I've seen before include "Undergo spasms (from pain or laughter)" , "Suffer involuntary contractions" .)