Silly vain English wet behind ears (5)
I believe the answer is:
naive
'wet behind ears' is the definition.
(I've seen this before)
'silly vain english' is the wordplay.
'silly' indicates anagramming the letters.
'english' becomes 'e' (abbreviation).
'vain' is an anagram of 'naiv'.
'naiv'+'e'='NAIVE'
(Other definitions for naive that I've seen before include "Excessively trusting or innocent" , "Wet behind ears" , "Simple-minded" , "Too trusting and impractical" , "Inexperienced, gullable" .)