My arrangement for strings and brass instrument (6)
I believe the answer is:
cornet
'brass instrument' is the definition.
(I've seen this before)
'my arrangement for strings' is the wordplay.
I cannot really see how this works, but
'my' could be 'cor' (both are expressions of surprise) and 'cor' is located in the answer.
The remaining letters 'net' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'and' acts as a link.
Can you help me to learn more?
(Other definitions for cornet that I've seen before include "Ice-cream - brass instrument" , "Brass musical instrument" , "its contents cold" , "Wind instrument" , "Trumpet-like instrument" .)