Numbers set out in rows (6)
I believe the answer is:
eights
'numbers' is the definition.
(eight is a kind of number)
'set out in rows' is the wordplay.
'set' becomes 'eight' (I have seen 'Set of rowers' mean 'eight' so perhaps 'set' could also mean 'eight').
'out in rows' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'eight'+'s'='EIGHTS'
Can you help me to learn more?
(Other definitions for eights that I've seen before include "Sets of rowers" , "Numbers for rowing crews" , "Boat race crews" , "Rowing boat race, eight eight" , "88888" .)