Familiar prisoner keeps it up (8)
I believe the answer is:
intimate
'familiar' is the definition.
(I've seen this before)
'prisoner keeps it up' is the wordplay.
'prisoner' becomes 'inmate' (inmate is a kind of prisoner**).
'keeps' means one lot of letters goes inside another.
'up' is a reversal indicator (in down clue: letters go upwards).
'it' backwards is 'ti'.
'inmate' placed around 'ti' is 'INTIMATE'.
(Other definitions for intimate that I've seen before include "Familiar; imply" , "Infer; personal" , "Give hint to close friend?" , "Close acquaintance" , "Hint to a familiar friend" .)