Ragamuffin, rather a scallywag at heart (6)
I believe the answer is:
rascal
'ragamuffin' is the definition.
(both can mean a dirty or naughty child)
'rather a scallywag at heart' is the wordplay.
'at heart' indicates a hidden word (letters hidden in the heart of the clue).
'RASCAL' can be found hidden inside 'rather a scallywag'.
(Other definitions for rascal that I've seen before include "A naughty chap or bold boy" , "Mischievous person or confused lascar" , "Mischievous fellow" , "not one to be admired" , "Pickle" .)