Westbound motorway cutting killed a deer maybe (7)
I believe the answer is:
animals
'deer maybe' is the definition.
'westbound motorway cutting killed a' is the wordplay.
'westbound' says the letters should be written in reverse (letters written from right to left).
'motorway' becomes 'M' (as in motorway names - Ms, Mi etc).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'killed' becomes 'slain' (slaying is a kind of killing).
'slain'+'a'='slaina'
'm' put inside 'slaina' is 'slamina'.
'slamina' written backwards gives 'ANIMALS'.
(Other definitions for animals that I've seen before include "Living things" , "pets maybe" , "Living beings" , "Living organisms characterised by voluntary movement" , "Wild beasts" .)