Strangely vain English easily conned (5)
I believe the answer is:
'easily conned' is the definition.
(I've seen this in another clue)
'strangely vain english' is the wordplay.
'strangely' is an anagram indicator.
'english' becomes 'e' (abbreviation as in OED).
'vain' with letters rearranged gives 'naiv'.
(Other definitions for naive that I've seen before include "Foolish, inexperienced" , "Unaffected or guileless" , "Rather too simple and gullible" , "Too credulous or trusting" , "Unaffectedly trusting" .)