Being deceitful is beneath worker (9)
I believe the answer is:
underhand
'being deceitful' is the definition.
(I've seen this before)
'beneath worker' is the wordplay.
'beneath' becomes 'under' (under means beneath or underneath).
'worker' becomes 'hand' (hand is a kind of worker).
'under'+'hand'='UNDERHAND'
'is' is the link.
(Other definitions for underhand that I've seen before include "Done in sly or secret way" , "Way of bowling" , "Done dishonestly" , "Deceitful" , "Surreptitious" .)