A great many boys have nothing (5)
I believe the answer is:
loads
'a great many' is the definition.
(I've seen this before)
'boys have nothing' is the wordplay.
'boys' becomes 'lads' (lad is a kind of boy).
'have' is an insertion indicator.
'nothing' becomes 'O' (O resembles 0).
'lads' going around 'o' is 'LOADS'.
(Other definitions for loads that I've seen before include "in quantity" , "Oodles (informal)" , "great deal" , "Charges; plenty" , "A great many" .)