Cloud user's terribly naive (9)
I believe the answer is:
'naive' is the definition.
(associated in meaning)
'cloud user's terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'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" .)