Soon Mousy's back to front and lost in the crowd (9)
I believe the answer is:
anonymous
'lost in the crowd' is the definition.
I can't tell whether this defines the answer.
'soon mousy's back to front' is the wordplay.
'soon' becomes 'anon' (synonyms).
'back' suggests the final letters.
'to front' becomes 'mous' (I can't justify this - if you can you should give a lot more credence to this answer).
The last letter of 'mousy' is 'y'.
'anon'+'y'+'mous'='ANONYMOUS'
'and' is the link.
Can you help me to learn more?
(Other definitions for anonymous that I've seen before include "Having no known name" , "Unidentified" , "unnamed source" , "Nameless" , "Of unknown identity" .)