Little Dorothy demonstrates a part of the Morse Code (3)
I believe the answer is:
dot
I believe this is a double definition.
'little dorothy' is the first definition.
(nickname for Dorothy)
'a part of the morse code' is the second definition.
('dot' is part of 'morse code')
'demonstrates' is the link.
(Other definitions for dot that I've seen before include "One signal used in Morse code" , "One of the signals in Morse code" , "Very small circular shape" , "Long-ago year?" , "Full stop, point" .)