Ahead, master swimming against the current (8)
I believe the answer is:
upstream
'against the current' is the definition.
('upstream' can be a synonym of 'against the current')
'ahead master swimming' is the wordplay.
'ahead' becomes 'up' ('up' can be similar in meaning to 'ahead').
'swimming' indicates anagramming the letters.
'master' is an anagram of 'stream'.
'up'+'stream'='UPSTREAM'
(Other definitions for upstream that I've seen before include "Going back to the source" , "at an earlier stage in process" , "Current trend is not" , "Towards (river's) source" , "Eats rump (anag)" .)