One posted to Yemeni port, briefly possessing fantastic des res (9)
I believe the answer is:
addressee
'one posted to' is the definition.
(someone who something has been posted to)
'yemeni port briefly possessing fantastic des res' is the wordplay.
'yemeni port' becomes 'Aden' (port in Yemen).
'briefly' means to remove the last letter.
'possessing' indicates putting letters inside.
'fantastic' indicates an anagram (a fantastic or improbable ordering of the letters).
'aden' with its final letter removed is 'ade'.
'des'+'res'='desres'
'desres' anagrammed gives 'dresse'.
'ade' enclosing 'dresse' is 'ADDRESSEE'.
(Other definitions for addressee that I've seen before include "Recipient of mail" , "Intended recipient of letter" , "One named on summons" , "The person to whom letter is delivered" , "One sent letter" .)