Primrose in part of animal’s mouth (7)
I believe the answer is:
cowslip
'primrose' is the definition.
(I know that cowslip is a type of primrose)
'part of animal's mouth' is the wordplay.
'part of animal' becomes 'cows' (I can't explain this - if you can you should believe this answer much more).
'mouth' becomes 'lip' (synonyms).
'cows'+'lip'='COWSLIP'
'in' is the link.
Can you help me to learn more?
(Other definitions for cowslip that I've seen before include "Yellow spring flower" , "Flower related to primrose" , "Plant with yellow flower" , "Wild flower of primrose family" , "Fictional rabbit" .)