Malicious gossip when Charlie appears in casual wear (7)
I believe the answer is:
scandal
'malicious gossip' is the definition.
(synonyms)
'charlie appears in casual wear' is the wordplay.
'charlie' becomes 'C' (phonetic alphabet).
'appears in' is an insertion indicator.
'casual wear' becomes 'sandal' (casual footwear).
'c' placed within 'sandal' is 'SCANDAL'.
'when' acts as a link.
(Other definitions for scandal that I've seen before include "Malicious gossip" , "Event causing public outrage" , "Something shameful or disgraceful, often public" , "Malicious talk; outrage" , "It injures reputation" .)