A person returning in Apocalypse Now (3)
I believe the answer is:
one
'a person' is the definition.
(an individual)
'returning in apocalypse now' is the wordplay.
'returning' is a reversal indicator.
'in' says the answer is hidden in the clue.
'eno' is hidden within 'apocalypse now'.
'eno' in reverse letter order is 'ONE'.
(Other definitions for one that I've seen before include "Sole; identical" , "1" , "joke" , "Single, unitary" , "Unit" .)