Draw river? Piece of cake! (4)
I believe the answer is:
tier
'piece of cake' is the definition.
(I've seen this in another clue)
'draw river?' is the wordplay.
'draw' becomes 'tie' (synonyms).
'river?' becomes 'r' (abbreviation for river).
'tie'+'r'='TIER'
(Other definitions for tier that I've seen before include "Rank - row" , "Layer, rank" , "I might draw" , "Storey" , "Rite for a rank" .)