Number in St Kitts regularly filling wharf (8)
I believe the answer is:
quantity
'number' is the definition.
(I've seen this before)
'in st kitts regularly filling wharf' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'filling' indicates putting letters inside.
'wharf' becomes 'quay' (quay is a kind of wharf).
'in'+'st'+'kitts'='instkitts'
The alternate letters of 'instkitts' are 'ntit'.
'ntit' put into 'quay' is 'QUANTITY'.
(Other definitions for quantity that I've seen before include "Amount or number" , "Person who costs construction work" , "Volume" , "Helping" .)