Dance number -- a few rock (9,4)
I believe the answer is:
eightsome reel
'dance' is the definition.
(I've seen this before)
'number a few rock' is the wordplay.
'number' becomes 'eight' (eight is a kind of whole number).
'a few' becomes 'some' (both can mean a small amount of something).
'rock' becomes 'reel' (reel can mean to rock around).
'eight'+'some'+'reel'='EIGHTSOME REEL'
(Other definitions for eightsome reel that I've seen before include "See leg hit more (anag) - Scottish dance" , "feature of ceilidh" , "Part of ceilidh perhaps" , "Lively Scottish dance" .)