A letter from Margaret in the outskirts of Ottawa (5)
I believe the answer is:
omega
'a letter' is the definition.
(I know that omega is a type of letter)
'margaret in the outskirts of ottawa' is the wordplay.
'margaret' becomes 'Meg' (short for Margaret).
'in the' indicates putting letters inside.
'outskirts of' says to hollow out the word (remove centre letters) (only letters on the outskirts).
'ottawa' with its centre removed is 'oa'.
'meg' placed within 'oa' is 'OMEGA'.
'from' acts as a link.
(Other definitions for omega that I've seen before include "Last thing" , "Greek finalist" , "Ultimate letter" , "Athenian character" , "long letter" .)