Express understands seedy nightspot's sudden decline (8)
I believe the answer is:
nosedive
'decline' is the definition.
(I know that nosedive is a type of decline)
'express understands seedy nightspot's' is the wordplay.
'express' indicates a 'sounds like' (homophone) clue (I've seen 'expressed' mean this).
'understands' becomes 'knows' (understand can mean to know or recognise).
'seedy nightspot' becomes 'dive' (I can't justify this - if you can you should give a lot more credence to this answer).
'knows' is a homophone of 'nose'.
'nose'+'dive'='NOSEDIVE'
'sudden' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for nosedive that I've seen before include "Take the plunge suddenly" , "steep fall" , "fall precipitately" , "drop sharply" , "Dramatic fall" .)