Labourer's assistant could be so devious (9)
I believe the answer is:
Here is my best explanation:
'devious' is the definition.
'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.
(Other definitions for underhand that I've seen before include "Secret and sly", "Done dishonestly", "In a secret or dishonest way", "delivery may be so made", "Sneaky", "Done in sly or secret way", "done deviously through cover", "Deceitful", "way of delivering service." and "sneaky".)