Blunt stunned -- small fine in prison! (9)
I believe the answer is:
outspoken
'blunt' is the definition.
('outspoken' can be a synonym of 'blunt')
'stunned small fine in prison' is the wordplay.
'stunned' becomes 'out' ('out' can be a synonym of 'stunned').
'small' becomes 's' (abbreviation - e.g. clothes size).
'fine' becomes 'ok' ('OK' can be a synonym of 'fine').
'in' is an insertion indicator.
'prison' becomes 'pen' (I've seen this before).
'ok' going within 'pen' is 'poken'.
'out'+'s'+'poken'='OUTSPOKEN'
(Other definitions for outspoken that I've seen before include "Blunt" , "bluff" , "Bold in speech" , "Describes one who always says what s/he thinks" , "not reserved?" .)