After reflection during Whitsun, I'm retiring -- end of the line (8)
I believe the answer is:
terminus
'end of the line' is the definition.
(I've seen this in another clue)
'after reflection during whitsun i'm retiring' is the wordplay.
'after reflection' shows that the letters should be reversed in order.
'during' says the answer is hidden in the clue.
'sunimret' is hidden in the letters of 'whitsun im retiring'.
'sunimret' reversed gives 'TERMINUS'.
(Other definitions for terminus that I've seen before include "End station" , "End of the line for tram or bus" , "bust revealed" , "Final stop" , "eg Paddington" .)