Ducks walk when crossing road (8)
I believe the answer is:
mallards
'ducks' is the definition.
(mallard is a kind of duck)
'walk when crossing road' is the wordplay.
'walk' becomes 'mall' (mall is a kind of walk**).
'when' becomes 'as'.
'crossing' means one lot of letters goes inside another.
'road' becomes 'rd' (abbreviation).
'as' going around 'rd' is 'ards'.
'mall'+'ards'='MALLARDS'
(Other definitions for mallards that I've seen before include "Wild dabbling ducks" , "LSD alarm for wild ducks" , "Slam lard badly on the ducks" , "Freshwater ducks" .)