Impassive girl many rejected (6)
I believe the answer is:
stolid
'impassive' is the definition.
(thesaurus)
'girl many rejected' is the wordplay.
'girl' becomes 'di' (**).
'many' becomes 'lots' (I've seen this before).
'rejected' says the letters should be written in reverse.
'di'+'lots'='dilots'
'dilots' in reverse letter order is 'STOLID'.
(Other definitions for stolid that I've seen before include "Wooden" , "Phlegmatic" , "Heavy" , "displaying little emotion" , "'Calm and dependable, showing little emotion (6)'" .)