Last letter from girl in borders of Oklahoma (5)
I believe the answer is:
omega
'last letter' is the definition.
(I've seen this before)
'girl in borders of oklahoma' is the wordplay.
'girl' becomes 'meg' (**).
'in' indicates putting letters inside.
'borders of' says to hollow out the word (remove centre letters).
'oklahoma' with its middle removed is 'oa'.
'meg' put inside 'oa' is 'OMEGA'.
'from' acts as a link.
(Other definitions for omega that I've seen before include "Last letter of the Greek alphabet" , "Greek long 'o'" , "Greek finalist" , "Final outcome" , "end of series" .)