Track opening of the bar (5)
I believe the answer is:
trail
'track' is the definition.
(I've seen this before)
'opening of the bar' is the wordplay.
'opening of' indicates taking the first letters.
'bar' becomes 'rail' (rail is a kind of bar).
The initial letter of 'the' is 't'.
't'+'rail'='TRAIL'
(Other definitions for trail that I've seen before include "Track, route" , "Wake" , "Stalk" , "Beaten path; be in losing position" , "Need to catch up with" .)