Biting fish eating river insect (9)
I believe the answer is:
trenchant
'biting' is the definition.
(I've seen this before)
'fish eating river insect' is the wordplay.
'fish' becomes 'tench' (tench is a kind of fish).
'eating' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation).
'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 "Vigorous, incisive" , "Distinctly defined" , "Acerbic" , "blunt" , "Clear-cut and hard-hitting" .)