Chopped a vein when wet behind the ears (5)
I believe the answer is:
naive
'wet behind the ears' is the definition.
(I've seen this in another clue)
'chopped a vein' is the wordplay.
'chopped' indicates an anagram (letters chopped or smashed up).
'a'+'vein'='avein'
'avein' anagrammed gives 'NAIVE'.
'when' acts as a link.
(Other definitions for naive that I've seen before include "Artless; simplistic" , "Too credulous or trusting" , "Inexperienced and guileless" , "Credulous, unworldly" , "Inexperienced or gullible" .)