Mammal’s milk, iodine rich primarily (5)
I believe the answer is:
tapir
'mammal's' is the definition.
(tapir is a kind of mammal)
'milk iodine rich primarily' is the wordplay.
'milk' becomes 'tap' (both can mean to exploit or use).
'primarily' indicates taking the first letters.
The first letters of 'iodine rich' is 'ir'.
'tap'+'ir'='TAPIR'
(Other definitions for tapir that I've seen before include "Browser" , "Nocturnal hoofed animal" , "A trip (anag) - animal" , "Animal with elongated snout" , "I part with the pig-like animal" .)