Retiring, some colleague I'd admired for so long (5)
I believe the answer is:
adieu
'for so long' is the definition.
(both can mean goodbye)
'retiring some colleague i'd admired' is the wordplay.
'retiring' shows that the letters should be reversed in order.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'ueida' is hidden within 'colleague id admired'.
'ueida' reversed gives 'ADIEU'.
(Other definitions for adieu that I've seen before include "Ciao, in French" , "Parting word" , "Auf wiedersehen" , "Vale" , "Leave" .)