Most important to net river and lake fish (6)
I believe the answer is:
marlin
'fish' is the definition.
(thesaurus)
'most important to net river and lake' is the wordplay.
'most important' becomes 'main' (I've seen this before).
'to net' is an insertion indicator.
'river' becomes 'r'.
'and' says to put letters next to each other.
'lake' becomes 'l'.
'r'+'l'='rl'
'main' enclosing 'rl' is 'MARLIN'.
(Other definitions for marlin that I've seen before include "Large sea-fish" , "Large ocean fish" , "type of fish" , "Spearfish" , "Large oceanic sport fish" .)