Mass of fish traversing lake (4)
I believe the answer is:
clod
'mass' is the definition.
(thesaurus)
'fish traversing lake' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish).
'traversing' means one lot of letters goes inside another.
'lake' becomes 'l'.
'cod' enclosing 'l' is 'CLOD'.
'of' acts as a link.
(Other definitions for clod that I've seen before include "Lump of turf" , "Lump of earth - stupid person" , "'Clump, glob (4)'" , "dope?" , "Block" .)