This might cut swan with broken neck if not caught (8)
I believe the answer is:
penknife
'this might cut' is the definition.
(a penknife can be used to cut things)
'swan with broken neck if not caught' is the wordplay.
I cannot quite see how this works, but
'swan' could be 'pen' (pen is a kind of swan) and 'pen' is found in the answer.
'if' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for penknife that I've seen before include "A Swiss army one has several blades and many tools" , "Handy little gadget" , "A Swiss army one is particularly handy" , "Small pocket cutting tool" , "whittling tool" .)