Second prisoner has one heir (5)
I believe the answer is:
scion
'heir' is the definition.
(I've seen this before)
'second prisoner has one' is the wordplay.
'second' becomes 's' (common abbreviation).
'prisoner' becomes 'con' (abbreviation for 'convict').
'has' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'con' placed around 'i' is 'cion'.
's'+'cion'='SCION'
(Other definitions for scion that I've seen before include "Branch" , "Young twig; descendant" , "Offshoot" , "one of a long line" , "Descendent - heir" .)