Left port, carrying full cargo (5)
I believe the answer is:
laden
'full cargo' is the definition.
(I've seen this before)
'left port carrying' is the wordplay.
'left' becomes 'l' (common abbreviation).
'port' becomes 'aden' (port in Yemen).
'carrying' means one lot of letters go next to another (in a down clue, the bottom letters carry others).
'l'+'aden'='LADEN'
(Other definitions for laden that I've seen before include "fully charged" , "packed with cargo" , "Like cargo boat" , "Encumbered" , "bearing burden" .)