Excuse could be so naive (7)
I believe the answer is:
evasion
'excuse' is the definition.
(I've seen this before)
'could be so naive' is the wordplay.
'could be' is an anagram indicator.
'so'+'naive'='sonaive'
'sonaive' anagrammed gives 'EVASION'.
(Other definitions for evasion that I've seen before include "deception" , "Act of avoidance or escape" , "prevaricating excuse" , "Avoidance of giving a direct answer" , "The avoidance of commitment" .)