Fish from lake caught by one out on it? (7)
I believe the answer is:
bloater
'fish' is the definition.
(I know that bloater is a type of herring)
'lake caught by one out on it?' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'caught' becomes 'oater' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one out' means to remove the last letter.
'on it?' says to put letters next to each other.
'by' with its last letter removed is 'b'.
'l'+'oater'='loater'
'loater' after 'b' is 'BLOATER'.
'from' acts as a link.
(Other definitions for bloater that I've seen before include "Something fishy" , "Partially dried herring" , "Buckling" , "Fish" , "Cured herring" .)