Mammal, rabbit, hid regularly amongst grass all around (5,3)
I believe the answer is:
fruit bat
'mammal' is the definition.
(fruit bat is a kind of mammal)
'rabbit hid regularly amongst grass all around' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'amongst' indicates putting letters inside (I've seen this in other clues).
'grass' becomes 'turf' (turf is grass).
'all around' says the letters should be written backwards.
'rabbit'+'hid'='rabbithid'
The alternate letters of 'rabbithid' are 'abti'.
'abti' inserted into 'turf' is 'tabtiurf'.
'tabtiurf' back-to-front is 'FRUIT-BAT'.
(Other definitions for fruit bat that I've seen before include "Long-snouted, large-eyed nectar feeder" , "Flying fox, for example" , "Large flying mammal" , "Flying fox; bait turf (anag.)" .)