Truncated opening feature (4)
I believe the answer is:
chin
'feature' is the definition.
(facial feature)
'truncated opening' is the wordplay.
'truncated' means to remove the last letter.
'opening' becomes 'chink' (chink is a kind of opening).
'chink' with its last letter taken off is 'CHIN'.
(Other definitions for chin that I've seen before include "Protruding feature" , "Outstanding jaw part" , "Jutting part of lower jaw" , "Facial feature" , "Lower face part" .)