Tramp at sea in Calais? That's bad news (6)
I believe the answer is:
bummer
'tramp' is the definition.
(thesaurus)
'sea in calais? that's bad news' is the wordplay.
I cannot really see how this works, but
'sea' could be 'mer' (I have seen 'French sea ' mean 'mer' so perhaps 'sea' could also mean 'mer') and 'mer' is present in the answer.
'bad' could be 'bum' and 'bum' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'at' acts as a link.
Can you help me to learn more?
(Another definition for bummer that I've seen is " Disappointing or unpleasant experience".)