City police seizing second runner's formal wear (4,3)

'formal wear' is the definition.
(formal wear worn by a soldier in the mess)

'city police seizing second runner's' is the wordplay.
'city police' becomes 'met' (short for the Metropolitan Police of London).
'seizing' is an insertion indicator.
'second' becomes 's' (common abbreviation).
'runner' becomes 'ski' (ski is a kind of runner).
's'+'ski'='sski'
'met' placed around 'sski' is 'MESS KIT'.