Has to assemble part of colonnade (4)
I believe the answer is:
stoa
'colonnade' is the definition.
(I've seen this before)
'has to assemble part' is the wordplay.
'part' indicates a hidden word.
'STOA' is hidden in the letters of 'has to assemble'.
'of' is the link.
(Other definitions for stoa that I've seen before include "Ancient Greek portico" , "Roofed walkway with colonnades - oats (anag)" , "Greek portico or colonnade" , "classical colonnade" , "See Greek walk" .)