Dance function with Grand Ball (5)
I believe the answer is:
tango
'dance' is the definition.
(Tango is a dance)
'function with grand ball' is the wordplay.
'function' becomes 'tan' (tangent, mathematical function).
'with' says to put letters next to each other.
'grand' becomes 'g' (G is short for grand as in a thousand).
'ball' becomes 'o' (O looks like a ball).
'tan'+'g'+'o'='TANGO'
(Other definitions for tango that I've seen before include "South American dance (for two?)" , "Modern ballroom dance" , "Movement from Argentina" , "One from Argentina" , "Dance of South American origin" .)