Aeroplane going round pole for a joke (4)
I believe the answer is:
jest
'for a joke' is the definition.
('jest' can be a synonym of 'joke')
'aeroplane going round pole' is the wordplay.
'aeroplane' becomes 'jet' (jet is a kind of aeroplane).
'going round' is an insertion indicator (some letters go around others).
'pole' becomes 's' (abbreviation for South, as in South Pole).
'jet' going around 's' is 'JEST'.
(Other definitions for jest that I've seen before include "` `Many a true word is spoken in . . . .' '" , "Banter, joke" , "Humorous remark" , "Quip" , "Joke, gag" .)