Assurance of girl, fifty (4)
I believe the answer is:
gall
'assurance' is the definition.
(thesaurus)
'girl fifty' is the wordplay.
'girl' becomes 'gal' (synonyms).
'fifty' becomes 'L' (Roman numeral).
'gal'+'l'='GALL'
'of' acts as a link.
(Other definitions for gall that I've seen before include "Bold impudent behaviour; abnormal growth on tree" , "This bladder is attached to the liver and stores bile" , "A sore spot or a bodily bladder" , "Audacity" , "Effrontery" .)