Policeman stops Europeans being snappy dressers (7)
I believe the answer is:
dandies
'snappy dressers' is the definition.
'policeman stops europeans' is the wordplay.
'policeman' becomes 'di' (I've seen this in other clues).
'stops' indicates putting letters inside.
'europeans' becomes 'danes' (Dane is an example).
'di' placed inside 'danes' is 'DANDIES'.
'being' is the link.
(Other definitions for dandies that I've seen before include "Blades" , "Men who pay too much attention to stylish clothes" , "bucks" , "Men who over-dress, like Beau Brummel" , "characters too trendy?" .)