Signature tune of the dancing gnomes (5,4)
I believe the answer is:
theme song
'signature tune' is the definition.
('theme song' can be a synonym of 'signature tune')
'the dancing gnomes' is the wordplay.
'dancing' is an anagram indicator.
'gnomes' is an anagram of 'mesong'.
'the'+'mesong'='THEME SONG'
'of' acts as a link.
(Other definitions for theme song that I've seen before include "Characteristic melody often repeated in musical" , "Signature tune" , "Give me Sunshine" .)