A sidekick bored by this writer’s bitterness (8)
I believe the answer is:
acrimony
'bitterness' is the definition.
(synonyms)
'a sidekick bored by this writer's' is the wordplay.
'sidekick' becomes 'crony' (synonyms).
'bored by' means one lot of letters goes inside another (inserted letters bore into the word).
'this writer's' becomes 'Im' (referring to the person writing the clues).
'crony' placed around 'im' is 'crimony'.
'a'+'crimony'='ACRIMONY'
(Other definitions for acrimony that I've seen before include "Bitterness of manner or words" , "I cry, moan (anag)" , "Bitterness of temper" , "Bad feeling" , "Asperity" .)