Father locks up a social outcast (6)
I believe the answer is:
pariah
'a social outcast' is the definition.
(I've seen this before)
'father locks up' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'locks' becomes 'hair' (locks can mean hair).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'hair' reversed gives 'riah'.
'pa'+'riah'='PARIAH'
(Other definitions for pariah that I've seen before include "Person ostracised by society" , "someone socially unacceptable" , "person shunned" , "Person despised and rejected by society" , "Person despised and avoided" .)