Up to the B in “beer” (4)
I believe the answer is:
'up to' is the definition.
(I've seen this in another clue)
'b in beer' is the wordplay.
'in' means one lot of letters goes inside another.
'beer' becomes 'ale' (type of beer).
'b' put inside 'ale' is 'ABLE'.
'the' is the link.
(Other definitions for able that I've seen before include "As capable as a mariner" , "Having the skills and qualifications required" , "See 10" , "See 21" , "accomplished" .)