Anonymous person having one for the road? (7)
I believe the answer is:
another
I believe this clue is a double definition.
'anonymous person' is the first definition.
(A.N. Other can mean an unspecified person)
'one for the road?' is the second definition.
(one for the road means another one)
'having' is the link.
(Other definitions for another that I've seen before include "Player or person not yet named or selected" , "Someone else" , "A further (item)" , "encore?" , "Additional, one more" .)