Clean, favourite child, endearingly (6)
I believe the answer is:
moppet
'child endearingly' is the definition.
(term of endearment for a child)
'clean favourite' is the wordplay.
'clean' becomes 'mop' (mopping is a kind of cleaning).
'favourite' becomes 'pet' (synonyms).
'mop'+'pet'='MOPPET'
(Other definitions for moppet that I've seen before include "Little darling" , "young child" , "Tot" , "Little child" , "The dear child met Pop" .)