Old-fashioned, essentially dull dance (3,4)
I believe the answer is:
pas seul
'dance' is the definition.
(pas seul is a kind of dance)
'old-fashioned essentially dull' is the wordplay.
'old-fashioned' becomes 'passe' ('passe' can be a synonym of 'old-fashioned').
'essentially' means to look at the middle letters.
The centre of 'dull' is 'ul'.
'passe'+'ul'='PAS SEUL'
(Other definitions for pas seul that I've seen before include "dance movement" , "dance without partner" , "Ballet solo" .)