Girl taking shower awfully composed? (8)
'awfully composed?' is the definition.
(compose can mean to write)
'girl taking shower' is the wordplay.
'girl' becomes 'miss' (synonyms).
'taking' means one lot of letters go next to another.
'shower' becomes 'pelt' (showers of rain may be said to pelt).
(Other definitions for misspelt that I've seen before include "Ecstacy,say", "literal?", "having written words wrongly", "having character faults?", "A character out, perhaps".)