A sentry abandoning attempt to consume last bit of Cornish pasty (5)
I believe the answer is:
ashen
'pasty' is the definition.
('ashen' can be a synonym of 'pasty')
'a sentry abandoning attempt to consume last bit of cornish' is the wordplay.
'abandoning' suggests deleting specific letters.
'attempt' becomes 'try' (I've seen this before).
'to consume' indicates putting letters inside.
'last bit of cornish' becomes 'h' (I am not sure about this - if you are sure you should believe this answer much more).
'sentry' with 'try' taken out is 'sen'.
'sen' going around 'h' is 'shen'.
'a'+'shen'='ASHEN'
(Other definitions for ashen that I've seen before include "Wan - wooden - pallid" , "lacking in colour" , "bleached" , "Anaemic-looking" , "Looking pale from illness" .)