Measures adult horses (7)
I believe the answer is:
amounts
'measures' is the definition.
(I know this)
'adult horses' is the wordplay.
'adult' becomes 'a' (abbreviation).
'horses' becomes 'mounts' (mount can mean a horse that is ridden).
'a'+'mounts'='AMOUNTS'
(Other definitions for amounts that I've seen before include "Totals, sums" , "Supplies" , "Total numbers or quantities" , "Measures" .)