Twice old guards left Iberian city (6)
I believe the answer is:
oporto
'iberian city' is the definition.
(I've seen this before)
'twice old guards left' is the wordplay.
'twice' says to double the letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'guards' means one lot of letters goes inside another (inserted letters are guarded).
'left' becomes 'port' (left side of a ship - opposite of starboard).
'o' duplicated is 'oo'.
'oo' enclosing 'port' is 'OPORTO'.
(Other definitions for oporto that I've seen before include "Portuguese wine trade centre" , "City near the mouth of the River Douro" , "Iberian city" , "Iberian location" , "Portugal's second city" .)