Crooked smile 'cos of grammatical error (8)
I believe the answer is:
solecism
'grammatical error' is the definition.
(thesaurus)
'crooked smile 'cos' is the wordplay.
'crooked' indicates anagramming the letters.
'smile'+'cos'='smilecos'
'smilecos' anagrammed gives 'SOLECISM'.
'of' acts as a link.
(Other definitions for solecism that I've seen before include "Error in grammar or manners" , "Grammatical (or social) blunder" , "Social gaffe" , "Absurd or mistaken word or act" , "A mistake in the use of language" .)