Silly person backing favourites in March (5-4)

'march' is the definition.
(goose step is a kind of march)

'silly person backing favourites' is the wordplay.
'silly person' becomes 'goose' (I've seen this in another clue).
'backing' shows that the letters should be reversed in order.
'favourites' becomes 'pets' ('pet' can be a synonym of 'favourite').
'pets' in reverse letter order is 'step'.
'goose'+'step'='GOOSE-STEP'

'in' acts as a link.