The last time doctor's coming round (5)
I believe the answer is:
omega
'the last' is the definition.
(I've seen this before)
'time doctor's coming round' is the wordplay.
'time' becomes 'age' (age is a kind of time).
'doctor' becomes 'mo' (medical officer).
'coming round' shows that the letters should be reversed in order.
'age'+'mo'='agemo'
'agemo' back-to-front is 'OMEGA'.
(Other definitions for omega that I've seen before include "Consummation" , "Last letter of the Greek alphabet" , ""The Alpha and the ___"" , "long letter" , "Ending of a series or sequence" .)