The Polish preposition w roughly translates to Eng. in, even though the distribution is different, as it is usual for prepositions. I'll give two examples of the use of said preposition:
(1) Narodowe muzeum morskie jest w Gdańsku. National museum marine is in Gdańsk. (2) Ja będę iść w czwartek. I will go on Thursday.
In (1) we have ['vgdaɲsku] for w Gdańsku and in (2) ['ftʃfartɛk] for w czwartek. This means that, like in many other cases in Polish, the last consonant of a word gets assimilated to the first consonant of the word which comes after it.
Now to the question: Should one consider w as a (pro)clitic?