Crack all right in aeroplane losing tail (4)
I believe the answer is:
joke
'crack' is the definition.
(I've seen this in another clue)
'all right in aeroplane losing tail' is the wordplay.
'all right' becomes 'ok' (synonyms).
'in' means one lot of letters goes inside another.
'aeroplane' becomes 'jet' (jet is a kind of aeroplane).
'losing tail' means to remove the last letter.
'jet' with its last letter taken off is 'je'.
'ok' placed within 'je' is 'JOKE'.
(Other definitions for joke that I've seen before include "Crack in a cracker" , "bit of humour" , "Humorous remark" , "Amusing remark" , "Amusing quip" .)