Unhappy note behind nightclub, hence tardy (12)
I believe the answer is:
disconsolate
'unhappy' is the definition.
(I've seen this before)
'note behind nightclub hence tardy' is the wordplay.
'note' becomes 'n' (abbreviation).
'behind' means one lot of letters go next to another.
'nightclub' becomes 'disco' (a disco is a club for dancing).
'hence' becomes 'so'.
'tardy' becomes 'late' (synonyms).
'n' after 'disco' is 'discon'.
'discon'+'so'+'late'='DISCONSOLATE'
(Other definitions for disconsolate that I've seen before include "Very sad or disappointed" , "Melancholy" , "Stuck with the blues" , "Woebegone" , "Low in spirits" .)