Runs behind simple boat (4)
I believe the answer is:
raft
'simple boat' is the definition.
(I've seen this before)
'runs behind' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'behind' becomes 'aft' ('aft' can be a synonym of 'behind').
'r'+'aft'='RAFT'
(Other definitions for raft that I've seen before include "A whole lot" , "large collection" , "Flat floating device" , "Floating framework" , "Flat water craft; great collection" .)