The coach takes us to Birkenhead (3)
I believe the answer is:
Here is my best explanation:
'the coach' is the definition.
('bus' can be a synonym of 'coach')
'us to birkenhead' is the subsidiary indication.
'to' means one lot of letters go next to another (I've seen this in other clues).
'birkenhead' becomes 'b' (head letter of 'birken').
'us' after 'b' is 'bus'.
'takes' is the link.
(Other definitions for bus that I've seen before include "Transport vehicle", "Mode of public transport", "People carrier", "Road vehicle able to carry many passengers", "Public transport vehicle", "Large passenger vehicle - small business!", "Coach", "Vehicle", "Use transport" and "Charabanc".)