Cutting fish, worker grasps flounder's tail (9)
I believe the answer is:
trenchant
'cutting' is the definition.
(I've seen this before)
'fish worker grasps flounder's tail' is the wordplay.
'fish' becomes 'tench' (tench is a kind of fish).
'worker' becomes 'ant' (worker is a type of ant).
'grasps' indicates putting letters inside.
'tail' indicates one should take the final letters.
The final letter of 'flounder' is 'r'.
'tench'+'ant'='tenchant'
'tenchant' enclosing 'r' is 'TRENCHANT'.
(Other definitions for trenchant that I've seen before include "Showing keenness" , "blunt" , "Incisive in expression" , "Forceful" , "Forthright, vigorous" .)