Collector and historian describing tanks containing moulded tin (11)
I believe the answer is:
antiquarian
'collector and historian' is the definition.
'describing tanks containing moulded tin' is the wordplay.
'describing tanks' becomes 'aquarian' (term meaning relating to aquariums).
'containing' means one lot of letters goes inside another.
'moulded' indicates an anagram.
'tin' anagrammed gives 'nti'.
'aquarian' going around 'nti' is 'ANTIQUARIAN'.
(Other definitions for antiquarian that I've seen before include "Collector of old things" , "dealing in rare books" , "Expert on old things" .)