Boys have nothing, or a great deal (5)
I believe the answer is:
loads
'a great deal' is the definition.
(I've seen this in another clue)
'boys have nothing' is the wordplay.
'boys' becomes 'lads' (lad is a kind of boy).
'have' is an insertion indicator.
'nothing' becomes 'O' (looks like zero - 0).
'lads' enclosing 'o' is 'LOADS'.
'or' acts as a link.
(Other definitions for loads that I've seen before include "Plenty (informal)" , "Tons, lots" , "Charges; plenty" , "very much" , "in quantity" .)