Transport plying back and forth (7)
I believe the answer is:
shuttle
'transport plying back and forth' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for shuttle that I've seen before include "It carries the horizontal thread in weaving" , "Transport plying to and fro" , "NASA space transport" , "It moves from side to side on the loom" , "Space vehicle" .)