Dance step is fluid going backwards (3)
I believe the answer is:
pas
'dance step' is the definition.
(I've seen this in another clue)
'fluid going backwards' is the wordplay.
'fluid' becomes 'sap' (fluid from plants).
'going backwards' is a reversal indicator.
'sap' back-to-front is 'PAS'.
'is' is the link.
(Other definitions for pas that I've seen before include "step on stage" , "Dance step" , "Dad's" , "Ballet duet" , "A bit of ballet" .)