Build some rectangular boxes (5)
I believe the answer is:
erect
'build' is the definition.
(erecting is a kind of building)
'some rectangular boxes' is the wordplay.
'boxes' indicates the answer is hidden within the clue (to box can mean to contain).
'ERECT' is hidden within 'some rectangular'.
(Other definitions for erect that I've seen before include "Standing straight" , "Crete (anag)" , "Assemble" , "Unbowed" , "Upstanding" .)