Of course, they won't require as much this afternoon, when son will stand in for daughter (8,2,3)
I believe the answer is:
needless to say
'of course' is the definition.
(I've seen this before)
'they won't require as much this afternoon when son will stand in for daughter' is the wordplay.
I cannot quite see how this works, but
'require' could be 'need' (synonyms) and 'need' is present in the answer.
'afternoon' could be 'a' (abbreviation) and 'a' is present in the answer.
'son' could be 'ss' (I've seen this in other clues) and 'ss' is located in the answer.
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for needless to say that I've seen is " Of course".)