Letter from old lady after short notice (5)
I believe the answer is:
sigma
'letter' is the definition.
(sigma is a kind of letter)
'old lady after short notice' is the wordplay.
'old lady' becomes 'ma'.
'after' means one lot of letters go next to another.
'short' means to remove the last letter.
'notice' becomes 'sign' (I've seen this before).
'sign' with its last letter taken away is 'sig'.
'ma' put after 'sig' is 'SIGMA'.
'from' acts as a link.
(Other definitions for sigma that I've seen before include "Eighteenth letter of the Greek alphabet" , "Agism of the Greek letter" , "character in Athens" , "Greek letter signifying sum in maths" , "Greek letter used as summation sign" .)