Ordinary seaman has to look after Belgian port (6)
I believe the answer is:
ostend
'belgian port' is the definition.
(I've seen this in another clue)
'ordinary seaman has to look after' is the wordplay.
'ordinary seaman' becomes 'os'.
'has' says to put letters next to each other.
'to look after' becomes 'tend' (tending is a kind of looking after).
'os'+'tend'='OSTEND'
(Other definitions for ostend that I've seen before include "Belgian ferry port" , "Port in Belgium" , "European port" , "Port -- stoned (anag)" , "Belgian port and resort" .)