Follow, heading off for bar (4)
I believe the answer is:
rail
'for bar' is the definition.
(rail is a kind of bar)
'follow heading off' is the wordplay.
'follow' becomes 'trail' (trail can mean to follow in secret).
'heading off' means to remove the first letter.
'trail' with its initial letter removed is 'RAIL'.
(Other definitions for rail that I've seen before include "Bar; waterbird" , "Complain about - marsh bird" , "Abuse, scold" , "A bird; bar to hang on" , "Scoff" .)