The old man's dancing step (3)
I believe the answer is:
pas
'dancing step' is the definition.
(a ballet step)
'the old man's' is the wordplay.
I cannot quite see how this works, but
'man' could be 'p' (abbreviation for pawn, a chess piece or 'man') and 'p' is found within the answer.
The remaining letters 'as' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for pas that I've seen before include "Office workers" , "A bit of ballet" , "step on stage" , "Type of sausage" , "... de deux, ballet step for two" .)