Biting fish catching river insect (9)
I believe the answer is:
trenchant
'biting' is the definition.
(thesaurus)
'fish catching river insect' is the wordplay.
'fish' becomes 'tench' (tench is a kind of fish**).
'catching' indicates putting letters inside.
'river' becomes 'r'.
'insect' becomes 'ant' (ant is a kind of insect).
'tench' enclosing 'r' is 'trench'.
'trench'+'ant'='TRENCHANT'
(Other definitions for trenchant that I've seen before include "Distinct" , "Vigorous and effective" , "Distinctly defined" , "Sharp" , "Forceful" .)