Mum left without a goodbye in the end (5)
I believe the answer is:
quiet
'mum' is the definition.
(thesaurus)
'left without a goodbye in the end' is the wordplay.
'left' becomes 'quit' (I've seen this in another clue).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'a goodbye in the end' becomes 'e' (I can't explain this - if you can you should give a lot more credence to this answer).
'quit' going around 'e' is 'QUIET'.
Can you help me to learn more?
(Other definitions for quiet that I've seen before include "Shh!" , "Free from activity" , "Silent" , "Silence, calm" , "Noiseless" .)