Both sons dismissed this non-stop balding (4,2,3)
I believe the answer is:
thin on top
'balding' is the definition.
(I've seen this before)
'both sons dismissed this non-stop' is the wordplay.
'both' becomes 'thi' (I can't justify this - if you can you should give a lot more credence to this answer).
'sons' becomes 's'.
'dismissed' suggests deleting specific letters.
'this non' becomes 'non'.
'non'+'stop'='nonstop'
'nonstop' with 's' removed is 'nontop'.
'thi'+'nontop'='THIN ON TOP'
Can you help me to learn more?
(Other definitions for thin on top that I've seen before include "with less than full head" , "Result of losing hair, balding" , "not so hairy now" , "showing scalp?" , "Balding (colloq.)" .)