Very old king I spotted in carriage (5)
I believe the answer is:
priam
'very old king' is the definition.
(mythological last king of Troy)
'i spotted in carriage' is the wordplay.
'spotted in' is an insertion indicator.
'carriage' becomes 'pram' (pram is a kind of carriage).
'i' put inside 'pram' is 'PRIAM'.
(Other definitions for priam that I've seen before include "Trojan king" , "Ruler of Troy" , "Old king" , "The last king of Troy, in Greek mythology" , "Old man of Paris" .)