Neither number has a rousing start! (3)
I believe the answer is:
nor
'neither' is the definition.
(I've seen this before)
'number has a rousing start' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'has' says to put letters next to each other.
'start' says to take the initial letters.
The initial letter of 'rousing' is 'r'.
'no'+'r'='NOR'
(Other definitions for nor that I've seen before include "It's also not" , "Conjunction meaning 'and not'" , "And not; Norman (abbr.)" , "And not either" , "Link word" .)