Son with affected mannerisms is a little rascal (5)
I believe the answer is:
scamp
'rascal' is the definition.
(synonyms)
'son with affected mannerisms is a little' is the wordplay.
'son' becomes 's'.
'with affected mannerisms' becomes 'camp' (camp can mean theatrical or affected).
'little' means to remove the last letter (the word is not quite complete).
's'+'camp'+'a'='scampa'
'scampa' with its final letter removed is 'SCAMP'.
(Other definitions for scamp that I've seen before include "Scallywag" , "Impish child" , "Mischievous child (colloq.)" , "Tearaway" , "Scoundrel" .)