Island's rocky outcrop spanning one mile (5)
I believe the answer is:
'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).
'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" .)