Heading for this lake to catch raw fish (5)
I believe the answer is:
trawl
'fish' is the definition.
(trawl can mean to fish)
'heading for this lake to catch raw' is the wordplay.
'heading for' suggests taking the first letters.
'to catch' is an insertion indicator.
The first letters of 'this lake' is 'tl'.
'tl' placed around 'raw' is 'TRAWL'.
(Other definitions for trawl that I've seen before include "Fishing-net" , "Use net" , "Net for fishing at sea" , "Fish using a towed net" , "fishing expedition" .)