Beset by loathing, son's hidden quickly (2,5)
I believe the answer is:
in haste
'quickly' is the definition.
(I've seen this before)
'beset by loathing son's hidden' is the wordplay.
'beset by loathing' becomes 'in hate' (loathing is hate).
'hidden' indicates putting letters inside.
'inhate' placed around 's' is 'IN HASTE'.
(Other definitions for in haste that I've seen before include "Rashly (how not to marry?)" , "Rushing round" , "Hurriedly" , "Quickly" , "Rapidly" .)