Rowers on Humber's first bit (6)
I believe the answer is:
eighth
'bit' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'rowers on humber's first' is the wordplay.
'rowers' becomes 'eight' (eight can mean a crew of eight rowers).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'humber's first' becomes 'h' (1st letter of 'humber').
'eight'+'h'='EIGHTH'
Can you help me to learn more?
(Other definitions for eighth that I've seen before include "The latest Henry or Edward?" , "Real fraction" , "Arm muscle" , "Twelve and a half percent" , "One after seventh" .)