Number of schooners (3)
I believe the answer is:
one
'number' is the definition.
(I know that one is a type of digit)
'of schooners' is the wordplay.
'of' says the answer is hidden in the clue.
'ONE' is hidden in the letters of 'schooners'.
(Other definitions for one that I've seen before include "Single and undivided" , "Sole; number" , "SECOND PART OF 16" , "1" , "This is singular" .)