Send back a large piece of lightweight material (5)
I believe the answer is:
balsa
'lightweight material' is the definition.
'send back a large piece' is the wordplay.
'send back' is a reversal indicator.
'large piece' becomes 'slab'.
'a'+'slab'='aslab'
'aslab' written backwards gives 'BALSA'.
'of' is the link.
(Other definitions for balsa that I've seen before include "Something light" , "South American tree" , "Tree yielding light wood" , "Very light timber" , "Wood used by model makers" .)