It's after the job's finished that there are offcuts (4)
I believe the answer is:
bits
'there are offcuts' is the definition.
I can't judge whether this defines the answer.
'it's after the job's finished' is the wordplay.
'after' means one lot of letters go next to another.
'the job's finished' becomes 'bi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ts' after 'bi' is 'BITS'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for bits that I've seen before include "Pieces, fragments" , "Parts" , "Small units of digital memory" , "Small pieces" .)