Look after a number, 500 (4)
I believe the answer is:
tend
'look after' is the definition.
(tending is a kind of looking after)
'a number 500' is the wordplay.
'a number' becomes 'ten' (ten is a kind of number**).
'500' becomes 'd' ('d' can be a synonym of '500').
'ten'+'d'='TEND'
(Other definitions for tend that I've seen before include "Care for, nurse" , "Minister" , "Head" , "Look after (someone)" , "Look after, or take certain direction" .)