Frank to reveal son all right in prison (9)
I believe the answer is:
outspoken
'frank' is the definition.
('outspoken' can be a synonym of 'frank')
'reveal son all right in prison' is the wordplay.
'reveal' becomes 'out' (outing is a kind of revealing**).
'son' becomes 's'.
'all right' becomes 'ok' ('ok' can be a synonym of 'all right').
'in' is an insertion indicator.
'prison' becomes 'pen' (I've seen this before).
'ok' placed into 'pen' is 'poken'.
'out'+'s'+'poken'='OUTSPOKEN'
'to' acts as a link.
(Other definitions for outspoken that I've seen before include "Candid" , "not reserved?" , "Inclined to say exactly what one thinks" , "bluff" , "Forthright, frank" .)