Cloud user's terribly naive (9)
I believe the answer is:
credulous
'naive' is the definition.
(associated in meaning)
'cloud user's terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'cloud'+'user'='clouduser'
'clouduser' with letters rearranged gives 'CREDULOUS'.
(Other definitions for credulous that I've seen before include "Apt to believe without sufficient evidence" , "Easily fooled" , "Too ready to believe things" , "Gullible" , "Likely to believe" .)