Quarrel about opening of motorcycle club - it's a source of bitterness (8)
I believe the answer is:
wormwood
'bitterness' is the definition.
(I've seen this before)
'quarrel about opening of motorcycle club' is the wordplay.
'quarrel' becomes 'row' (synonyms).
'about' says the letters should be written in reverse.
'opening of' suggests taking the first letters.
'club' becomes 'wood' (wood is a kind of club).
The first letter of 'motorcycle' is 'm'.
'row' backwards is 'wor'.
'wor'+'m'+'wood'='WORMWOOD'
'it's a source of' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for wormwood that I've seen before include "Woodworm found in kind of timber" , "(Plant used for) absinthe" , "Having the gall" , "Backward woodworm found in the herb" , "Herb used for absinthe" .)