Penetrating small mammal with end of wand (6)
I believe the answer is:
shrewd
'penetrating small' is the definition.
'shrewd' can be an answer for 'penetrating' (I've seen this before). I'm unsure of the 'small' bit.
'mammal with end of wand' is the wordplay.
'mammal' becomes 'shrew' (shrew is a kind of mammal).
'with end of wand' becomes 'd' (I can't justify this - if you can you should give a lot more credence to this answer).
'shrew'+'d'='SHREWD'
Can you help me to learn more?
(Other definitions for shrewd that I've seen before include "Craftily astute" , "Penetrating" , "Clever and calculating" , "Perspicacious" , "Astute, clever" .)