Dad locks up an outcast (6)
I believe the answer is:
pariah
'an outcast' is the definition.
('pariah' can be a synonym of 'outcast')
'dad locks up' is the wordplay.
'dad' becomes 'pa' ('pa' can be a synonym of 'dad').
'locks' becomes 'hair' (I've seen this before).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'hair' in reverse letter order is 'riah'.
'pa'+'riah'='PARIAH'
(Other definitions for pariah that I've seen before include "Outcast (state)" , "Outcast (dog)" , "someone socially unacceptable" , "Small box for valuables" , "Unacceptable person" .)