Late interval when dancing with unknown instead (13)
I believe the answer is:
alternatively
'instead' is the definition.
(synonyms)
'late interval when dancing with unknown' is the wordplay.
'when dancing with' is an anagram indicator (letters dance around).
'unknown' becomes 'y' (algebra).
'late'+'interval'='lateinterval'
'lateinterval' mixed up with 'y' gives 'ALTERNATIVELY'.
(Other definitions for alternatively that I've seen before include "Otherwise" , "On the other hand" , "as another option" .)