Measures adult horses, perhaps (7)
I believe the answer is:
amounts
'measures' is the definition.
(I know this)
'adult horses perhaps' is the wordplay.
I cannot really understand how this works, but
'adult' could be 'a' (abbreviation) and 'a' is located in the answer.
'horses' could be 'mounts' (mount can mean a horse that is ridden) and 'mounts' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for amounts that I've seen before include "Total numbers or quantities" , "Measures" , "Totals, sums" , "Supplies" .)