Prisoner, one fellow held by the French King (5)
I believe the answer is:
lifer
'prisoner' is the definition.
(lifer is a kind of prisoner)
'one fellow held by the french king' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'held by' means one lot of letters goes inside another.
'the french' becomes 'le'.
'king' becomes 'r' (r is abbreviation for rex).
'i'+'f'='if'
'if' placed into 'le' is 'life'.
'life'+'r'='LIFER'
(Other definitions for lifer that I've seen before include "convict given longest sentence" , "One with very long sentence" , "Convicted murderer, say" , "Serious criminal" , "Person serving long sentence" .)