He has a row over a ransom (7)
I believe the answer is:
oarsman
'he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a row over a ransom' is the wordplay.
'a row' indicates anagramming the letters.
'over' is a charade indicator (letters next to each other).
'a'+'ransom' is 'aransom'.
'aransom' with letters rearranged gives 'OARSMAN'.
'has' is the link.
Can you help me to learn more?
(Other definitions for oarsman that I've seen before include "galley slave, say" , "A male small boat propeller" , "It's bordered by Libya and France" , "A ransom (anag)" , "One in an eight?" .)