Head going off to purge press (4)
I believe the answer is:
urge
'press' is the definition.
('urge' can be a synonym of 'press')
'head going off to purge' is the wordplay.
'head going off to' means to remove the first letter.
'purge' with its initial letter taken off is 'URGE'.
(Other definitions for urge that I've seen before include "Incite to action" , "Impel - impulse" , "Press to take action" , "Yen" , "Press to do something" .)