Nail in rising water wears away (7)
I believe the answer is:
abrades
'wears away' is the definition.
(abrading is a kind of wearing away)
'nail in rising water' is the wordplay.
'nail' becomes 'brad' (a brad is a nail with rectangular cross-section).
'in' is an insertion indicator.
'rising' says the letters should be written in reverse (letters written upwards in a down clue).
'water' becomes 'sea' (the sea is water).
'sea' written backwards gives 'aes'.
'brad' put into 'aes' is 'ABRADES'.
(Another definition for abrades that I've seen is " Scratches".)