Gag son aboard plane (4)
I believe the answer is:
jest
'gag' is the definition.
(I know that gag can be written as jest)
'son aboard plane' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'aboard' means one lot of letters goes inside another.
'plane' becomes 'jet' (jet plane).
's' put inside 'jet' is 'JEST'.
(Other definitions for jest that I've seen before include "Jape, witticism" , "Sally" , "it's not serious, so to speak" , "Ridicule" , "Scoff; playfulness" .)