Portico partly opens to another (4)
I believe the answer is:
stoa
'portico' is the definition.
(thesaurus)
'partly opens to another' is the wordplay.
'partly' says the answer is hidden in the clue.
'STOA' is hidden within 'opens to another'.
(Other definitions for stoa that I've seen before include "See Greek walk" , "Oast (anag.)" , "Where Zeno spoke" , "(In Ancient Greece) walkway with a roof supported by colonnades" , "Roofed walkway with colonnades - oats (anag)" .)