Island crop provides a sign (9)
I believe the answer is:
capricorn
'a sign' is the definition.
(a sign of the zodiac)
'island crop' is the wordplay.
'island' becomes 'Capri' (Italian island).
'crop' becomes 'corn' (a farmer's crop of corn).
'capri'+'corn'='CAPRICORN'
'provides' is the link.
(Other definitions for capricorn that I've seen before include "Goat star sign" , "7 [SIGN OF THE ZODIAC]" , "Constellation known as The Goat" , "Tropic - Zodiac sign" , "Sign of winter" .)