A peak quantity (6)
I believe the answer is:
amount
'quantity' is the definition.
(synonyms)
'a peak' is the wordplay.
'peak' becomes 'mount' (I've seen this in another clue).
'a'+'mount'='AMOUNT'
(Other definitions for amount that I've seen before include "Quantity, sum" , "Sum of money" , "value" , "Supply" , "Total or extent" .)