Just about to slam dance (6)
I believe the answer is:
almost
'just about' is the definition.
('almost' can be a synonym of 'just about')
'to slam dance' is the wordplay.
'dance' indicates anagramming the letters.
'to'+'slam'='toslam'
'toslam' is an anagram of 'ALMOST'.
(Other definitions for almost that I've seen before include "Not entirely" , "practically" , "Not totally" , "Slightly short of" , "Well-nigh" .)