Fool in underwear during time inside home (9)
I believe the answer is:
birdbrain
'fool' is the definition.
(I've seen this before)
'underwear during time inside home' is the wordplay.
'underwear' becomes 'bra' (bra is a type of undergarment).
'during' is an insertion indicator.
'time inside' becomes 'bird' (slang for prison sentence).
'home' becomes 'in' ('he's home' can mean 'he's in').
'bird'+'in'='birdin'
'bra' put within 'birdin' is 'BIRDBRAIN'.
'in' acts as a link.
(Other definitions for birdbrain that I've seen before include "Fool" , "Stupid" , "Silly person (with sparrow's mind?)" , "Twittering twit?" .)