This is a bit obscure, but I'll have a go.
kayakamina's answer is, I'm sure, quite correct, so I'll carry on from there.
You are puzzled by "fish served in", but ignore "fish". The clue gives all the elements of the answer, then it says "served in", which means (I think) "offered up; presented; displayed for you", otherwise "these elements of the answer are here/presented to you in a vessel that emits vapour", which could be an organ that a clam uses to emit vapour.