Small quantity in narrow opening (4)
I believe the answer is:
slot
'narrow opening' is the definition.
(I've seen this in another clue)
'small quantity' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'quantity' becomes 'lot' (lot is a kind of quantity).
's'+'lot'='SLOT'
'in' is the link.
(Other definitions for slot that I've seen before include "Lots (anag.)" , "Fit into puzzle" , "Space for inserting coin" , "This gaming machine operates by insertion of coin" , "Scheduled time - hierarchical position" .)