Became less fooled when son replaced first daughter (8)
I believe the answer is:
dwindled
'became less' is the definition.
(to dwindle is to reduce or become less)
'fooled when son replaced first daughter' is the wordplay.
I cannot quite see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is located in the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is found in the answer.
'daughter' could be 'd' (genealogical abbreviation) and 'd' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for dwindled that I've seen before include "Grew less" , "Shrank" , "Became less by degrees" .)