Stage in endlessly severe horse race (2,5)
I believe the answer is:
st leger
'horse race' is the definition.
(British horserace)
'stage in endlessly severe' is the wordplay.
'stage' becomes 'leg' (synonyms).
'in' indicates putting letters inside.
'endlessly' means to remove the last letter.
'severe' becomes 'stern' (I've seen this before).
'stern' with its final letter taken away is 'ster'.
'leg' put into 'ster' is 'ST LEGER'.
(Other definitions for st leger that I've seen before include "Annual Doncaster race" , "Classic Doncaster horse race" , "Flat race" , "Annual horse-race for 3-yearolds at Doncaster" , "fixture for the flat" .)