Fish, angler's first to be caught in lake (5)
I believe the answer is:
loach
'fish angler's' is the definition.
'loach' can be an answer for 'fish' (I've seen this before). I am unsure of the 'angler's' bit.
'first to be caught in lake' is the wordplay.
'first' becomes 'a'.
'to be caught in' is an insertion indicator.
'lake' becomes 'loch' (loch is a kind of lake).
'a' put into 'loch' is 'LOACH'.
Can you help me to learn more?
(Other definitions for loach that I've seen before include "River fish - film director" , "Fish with barbels around mouth" , "Fish - British film director" , "Film-maker" , "Freshwater fish; rhymes with freshwater fish" .)