Charles is one man to catch rare crustacean (4,5)
I believe the answer is:
king prawn
'rare crustacean' is the definition.
'king prawn' can be an answer for 'crustacean' (I've seen this before). I am unsure of the 'rare' bit.
'charles is one man' is the wordplay.
'charles' becomes 'king' (Charles I or Charles II).
'is one man' becomes 'prawn' (I can't justify this - if you can you should give a lot more credence to this answer).
'king'+'prawn'='KING PRAWN'
'to catch' is the link.
Can you help me to learn more?
(Other definitions for king prawn that I've seen before include "Piece of seafood" , "crustacean" , "(Regal?) seafood" .)