Children belonging to start of fruitful season? (9)
I believe the answer is:
offspring
'children' is the definition.
(thesaurus)
'belonging to start of fruitful season?' is the wordplay.
'belonging to' becomes 'of' (I've seen this in other clues).
'start of' suggests taking the first letters.
'season?' becomes 'spring' (spring is a kind of season**).
The initial letter of 'fruitful' is 'f'.
'of'+'f'+'spring'='OFFSPRING'
(Other definitions for offspring that I've seen before include "Children, issue" , "Child" , "seed" , "Progeny" , "Young" .)