... one of two left over in Chequers originally (6)
I believe the answer is:
tropic
'one of two' is the definition.
(the Tropic of Cancer or Tropic of Capricorn)
'left over in chequers originally' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'over' says the letters should be written backwards.
'originally' suggests taking the first letters.
The first letters of 'in chequers' is 'ic'.
'port' in reverse letter order is 'trop'.
'trop'+'ic'='TROPIC'
(Other definitions for tropic that I've seen before include "Duck" , "Latitude associated with Capricorn or Cancer" , "Cancer/Capricorn line" , "Cancer is one, Capricorn another" , "Particular latitude" .)