Part of Sabbath used in this way (4)
I believe the answer is:
thus
'in this way' is the definition.
('thus' can be a synonym of 'in this way')
'part of sabbath used' is the wordplay.
'part of' says the answer is hidden in the clue.
'THUS' can be found hidden inside 'sabbath used'.
(Other definitions for thus that I've seen before include "As a result" , "It follows" , "an original Christmas gift" , "as indicated" , "Like so" .)