A sign that the island bears wheat (9)
I believe the answer is:
capricorn
'a sign' is the definition.
(a sign of the zodiac)
'the island bears wheat' is the wordplay.
'the island bears' becomes 'capri' (I can't explain this - if you can you should give a lot more credence to this answer).
'wheat' becomes 'corn' (synonyms).
'capri'+'corn'='CAPRICORN'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for capricorn that I've seen before include "Tropic - Zodiac sign" , "Goat star sign" , "It's a Tropic, and the Sign of the Goat" , "Constellation known as The Goat" , "7 [SIGN OF THE ZODIAC]" .)