Turning a hundred, I left line on map (6)
I believe the answer is:
tropic
'line on map' is the definition.
'turning a hundred i left' is the wordplay.
'turning' shows that the letters should be reversed in order.
'a hundred' becomes 'c' (Roman numeral).
'left' becomes 'port' (left side of a ship - opposite of starboard**).
'c'+'i'+'port'='ciport'
'ciport' reversed gives 'TROPIC'.
(Other definitions for tropic that I've seen before include "Duck" , "One of two parallels of latitude" , "hot spot" , "Latitude twenty-three degs twenty-six mins north or south" , "Cancer is one, Capricorn another" .)