A lot to teach boy about love (9)
I believe the answer is:
trainload
'a lot' is the definition.
(informal term for a large amount)
'to teach boy about love' is the wordplay.
'to teach' becomes 'train' (training is a kind of teaching).
'boy' becomes 'lad' (informal term for boy).
'about' means one lot of letters goes inside another.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'lad' going around 'o' is 'load'.
'train'+'load'='TRAINLOAD'
(Other definitions for trainload that I've seen before include "people aboard" , "all those expecting to be late?" , "Many commuters" , "Express content" .)