Means of staining poor-quality trousers, rather tight in front (8)
I believe the answer is:
dyestuff
'means of staining' is the definition.
(substance used for staining things)
'poor-quality trousers rather tight in front' is the wordplay.
'poor-quality' becomes 'duff' (duff can mean of poor quality).
'trousers' is an insertion indicator (to trouser can mean to steal or take).
'rather tight in front' becomes 'yest' (I can't explain this - if you can you should give a lot more credence to this answer).
'duff' enclosing 'yest' is 'DYESTUFF'.
(Other definitions for dyestuff that I've seen before include "Colourant" , "Anil perhaps" , "Substance used in staining" , "Colouring substance" , "Colouring agent" .)