A professional knife user holds this open large fish (8)
I believe the answer is:
sturgeon
'large fish' is the definition.
(I've seen this before)
'a professional knife user holds this open' is the wordplay.
'a professional knife user' becomes 'surgeon' (I can't justify this - if you can you should give a lot more credence to this answer).
'holds' is an insertion indicator.
'open' indicates taking the first letters (I've seen 'opens' mean this).
The first letter of 'this' is 't'.
'surgeon' placed around 't' is 'STURGEON'.
Can you help me to learn more?
(Other definitions for sturgeon that I've seen before include "Something fishy here" , "provider of delicacy?" , "So urgent to find caviar-bearing fish" , "supplier of delicacy" , "Fish from which caviar is obtained" .)