The tail end of daybreak on the mountain (7)
I believe the answer is:
everest
'mountain' is the definition.
(Mount Everest)
'the tail end of daybreak on the' is the wordplay.
'tail' indicates one should take the final letters.
'end of daybreak' becomes 'evers' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
The final letter of 'the' is 'e'.
'e' inserted into 'evers' is 'everes'.
'everes'+'t'='EVEREST'
Can you help me to learn more?
(Other definitions for everest that I've seen before include "Tee revs up the mountain" , "High peak" , "Tallest mountain ..." , "The highest mountain of all (7)" , "A high spot" .)