An examination of uplifting bit of circular oxygen (4)
I believe the answer is:
oral
'an examination' is the definition.
(oral is a kind of examination)
'uplifting bit of circular oxygen' is the wordplay.
'uplifting' shows that the letters should be reversed in order.
'bit of' indicates the answer is hidden within the clue.
'laro' can be found hidden inside 'circular oxygen'.
'laro' reversed gives 'ORAL'.
'of' acts as a link.
(Other definitions for oral that I've seen before include "Viva voce" , "language exam" , "Relating to the mouth - exam" , "Mouth-related" , "Spoken exam" .)