Geographical feature, centrepiece of grim area (3)
I believe the answer is:
ria
'geographical feature' is the definition.
'centrepiece of grim area' is the wordplay.
'centrepiece of' says to take the centre (piece from the middle).
'area' becomes 'a' (maths abbreviation).
The middle letters of 'grim' are 'ri'.
'ri'+'a'='RIA'
(Other definitions for ria that I've seen before include "Long narrow inlet" , "Institution for the promotion of art, in short" , "Coastal inlet" , "Air (anag.)" , "inlet between hills" .)