In lake there is a fish (5)
I believe the answer is:
loach
'fish' is the definition.
(loach is a kind of fish)
'in lake there' is the wordplay.
'in' indicates putting letters inside.
'lake' becomes 'loch' (loch is a kind of lake).
'there' becomes 'a' (I am not sure about this - if you are sure you should believe this answer much more).
'loch' placed around 'a' is 'LOACH'.
'is a' is the link.
Can you help me to learn more?
(Other definitions for loach that I've seen before include "Fish with barbels around mouth" , "Film-maker" , "Freshwater fish; rhymes with freshwater fish" , "River fish - film director" , "Fish - British film director" .)