For so long the last part of cavalcade follows right alongside (7)
I believe the answer is:
goodbye
'for so long' is the definition.
(synonyms)
'last part of cavalcade follows right alongside' is the wordplay.
I cannot really see how this works, but
'last' could be 'go' (synonyms) and 'go' is found in the answer.
'of' could be 'o' and 'o' is found in the answer.
'alongside' could be 'by' (I've seen this in other clues) and 'by' is found within the answer.
The remaining letters 'de' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Other definitions for goodbye that I've seen before include "Cheerio" , "Adieu" , "parting" , "so long" , "6 Down" .)