Narrowed gradually (7)
I believe the answer is:
tapered
'narrowed gradually' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tapered that I've seen before include "Becoming narrower towards one end" , "Predate (anag)" , "came to the point" , "Reducing to a final point" , "Gradually diminished" .)