Roman Emperor’s past it — usually not all there (5)
I believe the answer is:
titus
'roman emperor's' is the definition.
(Roman emperor 79 - 81 AD)
'past it usually not all there' is the wordplay.
'not all there' indicates the answer is hidden within the clue (only some of the letters are to be retained).
'TITUS' is hidden within 'past it usually'.
(Other definitions for titus that I've seen before include "Roman emperor - one of St Paul's correspondents" , "saint" , "-- Groan, novel by Mervyn Peake" , "& 20 Mervyn Peake character" , "identifying Philemon's neighbour" .)