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'm 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 explain this - if you can you should believe this answer much more).
'king'+'prawn'='KING PRAWN'
'to catch' acts as a 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" .)