Conscious feeling when missing motorway (8)
I believe the answer is:
sentient
'conscious' is the definition.
(similar in meaning)
'feeling when missing motorway' is the wordplay.
'feeling' becomes 'sentiment' (a sentiment is a feeling or view).
'when missing' indicates named letters should be taken away.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'sentiment' with 'm' taken away is 'SENTIENT'.
(Other definitions for sentient that I've seen before include "Capable of feeling things as in tenets" , "Feeling, conscious" , "Endowed with feeling and consciousness" , "Conscious - aware" , "Having feelings" .)