Bristle in anger at first when exhausted (3)
I believe the answer is:
awn
'bristle' is the definition.
(botanical term)
'anger at first when exhausted' is the wordplay.
'at first' says to take the initial letters.
'exhausted' suggests removing the centre (the word is emptied out or exhausted).
'when' with its middle taken out is 'wn'.
The first letter of 'anger' is 'a'.
'a'+'wn'='AWN'
'in' is the link.
(Other definitions for awn that I've seen before include "Beard of cereal" , "Bristle in flowering part of some grasses" , "Bristle on, eg, barley" , "bit of cereal" , "Bristle-like appendage on grass" .)