Without leader, window-fitter becomes less diligent (6)
I believe the answer is:
lazier
'less diligent' is the definition.
(diligent means hard-working or committed)
'without leader window-fitter' is the wordplay.
'without leader' means to remove the first letter.
'window fitter' becomes 'glazier' (I've seen this before).
'glazier' with its initial letter removed is 'LAZIER'.
'becomes' is the link.
(Other definitions for lazier that I've seen before include "More indolent or slothful" , "More idle" , "More lethargic or sluggish" , "less inclined to work" , "More like a couch potato" .)