Endlessly study court reply (5)
I believe the answer is:
react
'reply' is the definition.
(thesaurus)
'endlessly study court' is the wordplay.
'endlessly' means to remove the last letter.
'study' becomes 'read' ('read' can be a synonym of 'study').
'court' becomes 'ct'.
'read' with its last letter taken off is 'rea'.
'rea'+'ct'='REACT'
(Other definitions for react that I've seen before include "Change chemically" , "Respond emotionally" , "provide an answer" , "Show opposition" , "Act as opponent" .)