Start drinking surplus port (5)
I believe the answer is:
dover
'port' is the definition.
(I've seen this before)
'start drinking surplus' is the wordplay.
'start' indicates taking the first letters.
'surplus' becomes 'over' (I've seen this before**).
The initial letter of 'drinking' is 'd'.
'd'+'over'='DOVER'
(Other definitions for dover that I've seen before include "There'll be blue clouds over the white cliffs here" , "Sort of sole" , "White cliffs port" , "Channel port" , "The largest English ferry port" .)