Friend's promise to leave in the morning? (5)
I believe the answer is:
amigo
'friend's' is the definition.
('friend' in Spanish)
'promise to leave in the morning?' is the wordplay.
I cannot really understand how this works, but
'leave' could be 'go' (to go is to depart or leave) and 'go' is found in the answer.
'morning?' could be 'am' (as in 12:00 AM) and 'am' is found within the answer.
A single letter 'i' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for amigo that I've seen before include "Friend, comrade" , "Male (Spanish?) friend" , "Mexican friend" , "Quixote's friend" , "Friend in Spanish-speaking areas" .)