Lavish reward includes mammal (5)
I believe the answer is:
shrew
'mammal' is the definition.
(shrew is a kind of mammal)
'lavish reward includes' is the wordplay.
'includes' indicates the answer is hidden within the clue.
'SHREW' is hidden in the letters of 'lavish reward'.
(Other definitions for shrew that I've seen before include "Woman to be tamed?" , "eg Katharina" , "Small mouselike animal" , "difficult woman" , "Mouselike insectivore" .)