One flashing a blade in a row (9)
I believe the answer is:
oarswoman
'one flashing' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a blade in a row' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is located in the answer.
'blade' could be 'oar' (blade is the word for oar in competitive rowing) and 'oar' is present in the answer.
This explanation may well be incorrect...
(Other definitions for oarswoman that I've seen before include "Girl having paddle" , "River user" , "Rower" .)