Dancer, a Libra, Len upset (9)
I believe the answer is:
ballerina
'dancer' is the definition.
(ballerina is a kind of dancer)
'a libra len upset' is the wordplay.
'upset' indicates an anagram.
'a'+'libra'+'len'='alibralen'
'alibralen' is an anagram of 'BALLERINA'.
(Other definitions for ballerina that I've seen before include "agile performer" , "girl from the Kirov?" , "Pavlova?" , "girl on point duty" , "Danseuse" .)