A fat free demo, really! (9)
I believe the answer is:
literally
'really' is the definition.
(thesaurus)
'a fat free demo' is the wordplay.
'a fat free' becomes 'lite' (I can't explain this - if you can you should give a lot more credence to this answer).
'demo' becomes 'rally' (rally is a type of demonstration).
'lite'+'rally'='LITERALLY'
Can you help me to learn more?
(Other definitions for literally that I've seen before include "Without exaggeration" , "Not figuratively" , "According to the letter, exactly" , "Actually, really" , "Verbatim" .)