Labourer's assistant could be so devious (9)
I believe the answer is:
underhand
'devious' is the definition.
(thesaurus)
'labourer's assistant could be' is the wordplay.
I cannot really see how this works, but
'labourer' could be 'hand' (hand is a kind of labourer) and 'hand' is present in the answer.
The remaining letters 'under' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'so' acts as a link.
Can you help me to learn more?
(Other definitions for underhand that I've seen before include "Done in sly or secret way" , "delivery may be so made" , "In a secret or dishonest way" , "might be deceptive" , "Done dishonestly" .)