First of all, horses must be given numbers (7)
I believe the answer is:
amounts
'numbers' is the definition.
(I know this)
'first of all horses must be given' is the wordplay.
'first of all' becomes 'a' (1st letter of 'all').
'horses' becomes 'mounts' (mount is a kind of horse).
'must be given' is a charade indicator (letters next to each other).
'a'+'mounts'='AMOUNTS'
(Other definitions for amounts that I've seen before include "Supplies" , "Total numbers or quantities" , "Totals, sums" , "Measures" .)