Dance is hard work (3)
I believe the answer is:
hop
'dance' is the definition.
(hop is a kind of dance)
'hard work' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'work' becomes 'op' (abbreviation for opus).
'h'+'op'='HOP'
'is' is the link.
(Other definitions for hop that I've seen before include "..., skip and jump" , "It's used in brewing" , "Jump on a pogo stick" , "Climbing plant used to flavour beer (3)" , "Jump about on one leg" .)