When Uncle's on the plane, this is thrown out (6)
I believe the answer is:
jetsam
'plane this is thrown out' is the definition.
The definition suggests a singular noun which matches the answer.
'when uncle's on the' is the wordplay.
'when' becomes 'je' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'uncle' becomes 'sam' (Uncle Sam is an example).
'on' means one lot of letters go next to another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'sam' after 't' is 'tsam'.
'je'+'tsam'='JETSAM'
Can you help me to learn more?
(Other definitions for jetsam that I've seen before include "Discarded rubbish" , "Unwanted stuff thrown overboard" , "Flotsam and --" , "Cargo cast overboard" , "Rubbish on shore thrown from ships" .)