Stretching son with schooling (9)
I believe the answer is:
straining
'stretching' is the definition.
(thesaurus)
'son with schooling' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'with' is a charade indicator (letters next to each other).
'schooling' becomes 'training' (training is a kind of schooling).
's'+'training'='STRAINING'
(Other definitions for straining that I've seen before include "Exerting much effort or energy" , "demanding" , "Making a great effort" .)