Doctors with small underwear (5)
I believe the answer is:
vests
'doctors with small underwear' is the definition.
The definition and answer can be both related to bodily activities as well as being verbs in their -s form.
Perhaps there's a link between them I don't understand?
'doctors with small' is the wordplay.
'doctors' becomes 'vets' (vet is a kind of doctor).
'with' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vets' going around 's' is 'VESTS'.
Can you help me to learn more?
(Another definition for vests that I've seen is " Sleeveless garments".)