Relations going to the toilet - short visit (4-2)
I believe the answer is:
look-in
'short visit' is the definition.
(I've seen this before)
'relations going to the toilet' is the subsidiary indication.
'relations' becomes 'kin' (I've seen this in other clues**).
'going to' means one lot of letters go next to another.
'the toilet' becomes 'loo' (loo is a kind of toilet).
'kin' put after 'loo' is 'lookin'.
(Other definitions for look-in that I've seen before include "Watch TV (colloq.)" , "Chance of success" , "Make a short visit of visual sort?" .)