Island's rocky outcrop spanning one mile (5)
I believe the answer is:
timor
'island's' is the definition.
(I know that Timor is a islands)
'rocky outcrop spanning one mile' is the wordplay.
'rocky outcrop' becomes 'tor' (term for a hill).
'spanning' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'mile' becomes 'm' (common abbreviation).
'i'+'m'='im'
'tor' placed around 'im' is 'TIMOR'.
(Other definitions for timor that I've seen before include "East Indies island NW from Darwin" , "Island of the Malay Archipelago" , "Eastern island" , "SE Asian island" , "East . . . . ., became independent of Indonesia in 2002" .)