Very fast body of water (6)
I believe the answer is:
solent
'body of water' is the definition.
(I've seen this before)
'very fast' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'so'+'lent'='SOLENT'
(Other definitions for solent that I've seen before include "Channel north of IOW" , "Channel north of the Isle of Wight" , "body of water" , "stretch of water" , "Channel N. of IOW" .)