Think lake will contain small shellfish (6)
I believe the answer is:
mussel
'shellfish' is the definition.
(mussel is a kind of shellfish)
'think lake will contain small' is the wordplay.
'think' becomes 'muse' (musing is a kind of thinking).
'lake' becomes 'l'.
'will contain' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'muse'+'l'='musel'
'musel' placed around 's' is 'MUSSEL'.
(Other definitions for mussel that I've seen before include "Bivalve with purple-black shell" , "Black marine bivalve" , "Edible bivalve mollusc, common in sea" , "Edible marine mollusc sold by Molly Malone - not the cockle" , "Edible shellfish" .)