Letter from Greece has note for mother to reflect upon (5)
I believe the answer is:
gamma
'letter from greece' is the definition.
(letter from the Greek alphabet)
'mother to reflect upon' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'to' means one lot of letters go next to another (I've seen this in other clues).
'reflect upon' becomes 'gam' (I can't justify this - if you can you should believe this answer much more).
'ma' put after 'gam' is 'GAMMA'.
'has note for' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for gamma that I've seen before include "Foreign character" , "A bit of Greek" , "third highest grade" , "The Greek g, third in alphabet" , "Greek character" .)