Yes, seen rarely to possess intelligence — I can vouch for it (10)
I believe the answer is:
eyewitness
'i can vouch for it' is the definition.
(an eyewitness can vouch that something happened)
'yes seen rarely to possess intelligence' is the wordplay.
I cannot really see how this works, but
'yes' could be 'y' and 'y' is present in the answer.
'intelligence' could be 'wit' (wit is a kind of intelligence) and 'wit' is found within the answer.
an anagram of 'seen' is 'enes' which is present in the remaining letters.
The remaining letters 'es' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for eyewitness that I've seen before include "One seeing (eg, crime)" , "Onlooker" , "The Observer" , "He observed" , "evidence?" .)