Count upsetting a large posh club maybe (7)
I believe the answer is:
dracula
'count' is the definition.
(Count Dracula)
'upsetting a large posh club maybe' is the wordplay.
'upsetting' says the letters should be written in reverse.
'large' becomes 'L' (eg in clothes sizes).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'club maybe' becomes 'card' (clubs is a playing card suit).
'a'+'l'+'u'+'card'='alucard'
'alucard' back-to-front is 'DRACULA'.
(Other definitions for dracula that I've seen before include "Famous vampire" , "gothic novel" , "Bram Stoker creation" , "Literary vampire" , "Fictional vampire" .)