Henry apt to imprison wife, the idiot! (4-3)
I believe the answer is:
half-wit
'the idiot' is the definition.
('half-wit' can be a synonym of 'idiot')
'henry apt to imprison wife' is the wordplay.
'henry' becomes 'hal'.
'apt' becomes 'fit' (I've seen this before).
'to imprison' means one lot of letters goes inside another.
'wife' becomes 'w'.
'fit' going around 'w' is 'fwit'.
'hal'+'fwit'='HALF-WIT'
(Other definitions for half-wit that I've seen before include "Fool" , "One only 50% all there?" , "Stupid person" , "Hit flaw (anag.)" , "Idiot" .)