Number that's in excess (6)
I believe the answer is:
ninety
This clue could be a double definition.
'number' is the first definition.
(ninety is a kind of whole number)
'in excess' is the second definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?
'that's' is the link.
(Other definitions for ninety that I've seen before include "Four score and ten" , "Joe perhaps" , "Fourscore and ten" , "quite an age" , "Minutes played in a football match" .)