Building a pickup van (5)
I believe the answer is:
tower
'building' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for tower that I've seen before include "One to pull" , "Campanile, spire etc" , "famous prison" , "See 4 Down" , "Turret" .)