Subcontinent shown in main diagram (5)
I believe the answer is:
india
'subcontinent' is the definition.
(the Indian subcontinent)
'shown in main diagram' is the wordplay.
'shown in' indicates a hidden word.
'INDIA' is hidden in the letters of 'main diagram'.
(Other definitions for india that I've seen before include "Asian sub-continent" , "Sub-continental country" , "South Asian republic" , "IDA in the country?" , "A Passage to - (E.M. Forster)" .)