Char swimming round old fish (5)
I believe the answer is:
'fish' is the definition.
(roach is a kind of fish)
'char swimming round old' is the wordplay.
'swimming' is an anagram indicator.
'round' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'char' with letters rearranged gives 'rach'.
'rach' enclosing 'o' is 'ROACH'.
(Other definitions for roach that I've seen before include "member of family carp" , "Trim" , "Silver and red freshwater fish" , "Something fishy here" , "creepy-crawly" .)