Father and son dance (3)
I believe the answer is:
pas
'dance' is the definition.
(I've seen this before)
'father and son' is the wordplay.
'father' becomes 'pa' (pa is a kind of father).
'and' means one lot of letters go next to another.
'son' becomes 's'.
'pa'+'s'='PAS'
(Other definitions for pas that I've seen before include "step on stage" , "Dancing step" , "Office workers" , "Dance movement" , "... de deux, ballet step for two" .)