Angry at being collared by bore when heading off (5)
I believe the answer is:
irate
'angry' is the definition.
(irate can mean angry or annoyed)
'at being collared by bore when heading off' is the wordplay.
'being collared by' is an insertion indicator (I've seen this in other clues).
'bore' becomes 'tire' (I've seen this in other clues).
'when heading off' means to remove the first letter.
'tire' with its first letter taken off is 'ire'.
'at' placed into 'ire' is 'IRATE'.
(Other definitions for irate that I've seen before include "Angry and incensed" , "Mad" , "Fuming" , "Cross, irritated" , "Angry, annoyed" .)