Left port heavily loaded (5)
I believe the answer is:
laden
'heavily loaded' is the definition.
(I've seen this before)
'left port' is the wordplay.
'left' becomes 'l' (common abbreviation).
'port' becomes 'aden' (I've seen this before**).
'l'+'aden'='LADEN'
(Other definitions for laden that I've seen before include "Heavily weighed down" , "Carrying goods" , "Heavily loaded" , "Burdened, loaded" , "Weighed down with much cargo" .)