The old aircraft arrived, then left (5)
I believe the answer is:
camel
'the old aircraft' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'arrived then left' is the wordplay.
'arrived' becomes 'came' ('come' can be a synonym of 'arrive').
'then' is a charade indicator (letters next to each other).
'left' becomes 'l' (common abbreviation).
'came'+'l'='CAMEL'
Can you help me to learn more?
(Other definitions for camel that I've seen before include "Cud-chewing mammal of desert regions" , "African transport" , "Desert ruminant" , "Old WWI plane" , "Dromedary, perhaps" .)