Skilled worker, one devoted to a cause, heading off (7)
I believe the answer is:
artisan
'skilled worker' is the definition.
(an artisan is a skilled craftsperson)
'one devoted to a cause heading off' is the wordplay.
'one devoted to a cause' becomes 'partisan' (supporter or fighter for a particular cause).
'heading off' means to remove the first letter.
'partisan' with its initial letter removed is 'ARTISAN'.
(Other definitions for artisan that I've seen before include "Tsarina (anag.)" , "Skilled worker or craftsman" , "Skilled craftsperson" , "Skilled workman" , "Technician" .)