One nagging smallest mammal (5)

I believe the answer is:

shrew

I'm a little stuck... Click here to teach me more about this clue!

'mammal' is the definition.
(I know that shrew is a type of insectivore)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for shrew that I've seen before include "Long-nosed mammal" , "insect eater" , "Scold; insectivore" , "Virago" , "Insectivorous mammal" .)

I've seen this clue in The Sun.

Want a hint initially instead of a full solution? Install my app