Plain in part of Cornwall, an observatory (5)
I believe the answer is:
llano
'plain' is the definition.
(a grassy plain)
'part of cornwall an observatory' is the wordplay.
'part of' says the answer is hidden in the clue.
'LLANO' can be found hidden inside 'cornwall an observatory'.
'in' is the link.
(Other definitions for llano that I've seen before include "Grassy plain in South?America" , "(South American) grassy plain" , "Treeless area" , "Treeless grassy plain in South America" , "Flat area" .)