An infantile thing to do (6)
I believe the answer is:
teethe
'do' is the definition.
The answer and definition can be both related to bodily activities as well as being verbs in their base form.
Maybe there's a link between them I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for teethe that I've seen before include "Of baby, having first gum growth" , "Cut one's first ivories" , "Experience growing pains" , "Develop gnashers" , "(Of baby) have gum pain" .)