Animal doctor holding son’s garment (4)
I believe the answer is:
vest
'garment' is the definition.
(vest is a type of garment or item of clothing)
'animal doctor holding son's' is the wordplay.
'animal doctor' becomes 'vet' (veterinarian).
'holding' is an insertion indicator.
'vet' placed around 's' is 'VEST'.
(Other definitions for vest that I've seen before include "Clothe in ecclesiastical garments" , "Upper undergarment, singlet" , "Waistcoat (in USA)" , "Sleeveless undergarment" , "An American man's waistcoat" .)