Mother and boy start yielding to an ailment (6)
I believe the answer is:
malady
'an ailment' is the definition.
(I've seen this before)
'mother and boy start yielding' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'and' means one lot of letters go next to another.
'boy' becomes 'lad' (informal term for boy).
'start' suggests taking the first letters.
The first letter of 'yielding' is 'y'.
'ma'+'lad'+'y'='MALADY'
'to' is the link.
(Other definitions for malady that I've seen before include "Sickness or disease" , "affection" , "Complaint" , "Disease or disorder" , "hay fever, perhaps" .)