Creature — insect — found in motorway services principally (6)
I believe the answer is:
mantis
'creature' is the definition.
(mantis is a kind of creature)
'insect found in motorway services principally' is the wordplay.
'insect' becomes 'ant' (ant is a kind of insect**).
'found in' means one lot of letters goes inside another.
'motorway' becomes 'mi' (resembles Ma, English motorway).
'principally' indicates taking the first letters.
The first letter of 'services' is 's'.
'ant' inserted within 'mi' is 'manti'.
'manti'+'s'='MANTIS'
(Other definitions for mantis that I've seen before include "A praying . . . . . . is a kind of grasshopper" , "Preying animal, the female of which is deadlier than the male!" , "Insect in attitude of prayer" , "Room equipped for milking cows" , "Praying ..., long-bodied, large-eyed insect" .)