Bit of rubble in tradesman's waste (7)
I believe the answer is:
fritter
'waste' is the definition.
(frittering is a kind of wasting)
'bit of rubble in tradesman's' is the wordplay.
'bit of' indicates taking the first letters.
'in' is an insertion indicator.
'tradesman' becomes 'fitter' (someone who fits things).
The initial letter of 'rubble' is 'r'.
'r' placed inside 'fitter' is 'FRITTER'.
(Other definitions for fritter that I've seen before include "Hot food" , "Piece of meat, etc., coated in batter and fried" , "blow" , "bit of batter" , "Battered item" .)