Charles keeps in fetters! (6)
I believe the answer is:
chains
'fetters' is the definition.
(I've seen this before)
'charles keeps in' is the wordplay.
'charles' becomes 'chas' (shortened form).
'keeps' is an insertion indicator.
'chas' going around 'in' is 'CHAINS'.
(Other definitions for chains that I've seen before include "Restraints; series" , "Restriction" , "help to pull?" , "Cash in (anag)" , "Metal ropes with links (6)" .)