Dandy in a vehicle on entering M1 (8)
I believe the answer is:
macaroni
'dandy' is the definition.
(I've seen this before)
'a vehicle on entering m1' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'entering' is an insertion indicator.
'm1' becomes 'mi' (resembles M1).
'a'+'car'+'on'='acaron'
'acaron' inserted into 'mi' is 'MACARONI'.
'in' acts as a link.
(Other definitions for macaroni that I've seen before include "Thin tubes of pasta" , "food at trattoria?" , "finishes off burlesque verse" , "Slender tubes of pasta" , "Type of pasta" .)