Someone in boat, one having an argument? (5)
I believe the answer is:
rower
This clue might be a double definition.
'someone in boat' is the first definition.
(someone in a rowing boat)
'one having an argument?' is the second definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
Can you help me to learn more?
(Other definitions for rower that I've seen before include "Oarsman/woman" , "One who argues" , "Person using 9" , "One in eight, perhaps" , "Maybe one in eight" .)