Modest nun pirouettes in disguise (13)
I believe the answer is:
unpretentious
'modest' is the definition.
(thesaurus)
'nun pirouettes in disguise' is the wordplay.
'in disguise' indicates an anagram.
'nun'+'pirouettes'='nunpirouettes'
'nunpirouettes' is an anagram of 'UNPRETENTIOUS'.
(Other definitions for unpretentious that I've seen before include "airless?" , "Without airs and graces" .)