Some ailing rowers have top of nail extending abnormally into the flesh (7)
I believe the answer is:
ingrown
'some ailing rowers have top of nail extending abnormally' is the definition.
I can't tell whether this definition defines the answer.
'into the flesh' is the wordplay.
'into' becomes 'in'.
'the flesh' becomes 'grown' (I can't justify this - if you can you should give a lot more credence to this answer).
'in'+'grown'='INGROWN'
Can you help me to learn more?
(Other definitions for ingrown that I've seen before include "Worn gin (anag.)" , "Like some toenails" , "with innate quality" , "Innate; inward-looking" , "misplaced nail" .)