Follow, when he leaves the bar (5)
I believe the answer is:
trail
'follow' is the definition.
(trail can mean to follow in secret)
'he leaves the bar' is the wordplay.
'leaves' indicates named letters should be taken away.
'bar' becomes 'rail' (rail is a kind of bar).
'the' with 'he' taken out is 't'.
't'+'rail'='TRAIL'
'when' is the link.
(Other definitions for trail that I've seen before include "Be drawn belong behind" , "Beaten path; track" , "Beaten path; follow" , "Dog - track" , "Rural footpath" .)