A sort of dance? Move slowly if necessary (2,1,5)
I believe the answer is:
at a pinch
'if necessary' is the definition.
(I've seen this before)
'a sort of dance? move slowly' is the wordplay.
'sort of dance?' becomes 'tap' (I've seen this before).
'move slowly' becomes 'inch' (I've seen this in another clue).
'a'+'tap'+'inch'='AT A PINCH'
(Other definitions for at a pinch that I've seen before include "In case of necessity" , "If really necessary" , "under pressure" , "in a crisis?" , "If absolutely necessary" .)