Many love young man's trousers (5)
I believe the answer is:
loads
'many' is the definition.
'love young man's trousers' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'young man's' becomes 'lad's' (lad can mean a boy or young man).
'trousers' means one lot of letters goes inside another (trouser can mean to steal or take).
'o' placed within 'lads' is 'LOADS'.
(Other definitions for loads that I've seen before include "bags" , "Tons, lots" , "Cargoes; lots" , "Charges; plenty" , "Tons, oodles" .)