Very fast stretch of water (6)
I believe the answer is:
solent
'stretch of water' is the definition.
(I've seen this before)
'very fast' is the wordplay.
'very' becomes 'so' (I've seen this in other clues).
'fast' becomes 'lent' (I've seen this before).
'so'+'lent'='SOLENT'
(Other definitions for solent that I've seen before include "stretch of water" , "Channel separating Hampshire from the Isle of Wight" , "Water to north of IOW" , "Channel N. of IOW" , "Lets on to be an English waterway" .)