Instrument with which to try to catch fish? (8)
I believe the answer is:
castanet
'instrument with which to try to catch fish?' is the definition.
'castanet' can be an answer for 'instrument' (castanet is a kind of instrument). I am unsure of the rest of the definition.
'instrument with which' is the wordplay.
'instrument' becomes 'cat' (cat is a kind of instrument).
'with' means one lot of letters goes inside another.
'which' becomes 'stane' (I can't justify this - if you can you should give a lot more credence to this answer).
'cat' placed around 'stane' is 'CASTANET'.
(Other definitions for castanet that I've seen before include "something resembling a shell" , "One of a pair in Spanish music" , "musical accompaniment?" , "Single Spanish percussion" , "instrument" .)