Who mostly takes in a son's laundry? (4)
I believe the answer is:
wash
'laundry?' is the definition.
('wash' can be a synonym of 'laundry')
'who mostly takes in a son's' is the wordplay.
'mostly' means to remove the last letter.
'takes in' indicates putting letters inside.
'who' with its last letter removed is 'wh'.
'a'+'s'='as'
'wh' going around 'as' is 'WASH'.
(Other definitions for wash that I've seen before include "bath" , "Clean; dilute coat" , "Be credible" , "The ----, inlet of the North Sea" , "Of animals, cleanse by licking" .)