Shell movers (4)
I believe the answer is:
oars
'shell movers' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oars that I've seen before include "Implements to propel a boat" , "Rowing blades" , "Implements of propulsion on water" , "Soar around the rowers" , "Rowing equipment" .)