A London vagrant sheltering in church portico (9)
I believe the answer is:
colonnade
'portico' is the definition.
(thesaurus)
'a london vagrant sheltering in church' is the wordplay.
'vagrant' indicates anagramming the letters (I've seen this in other clues).
'sheltering' means one lot of letters goes inside another.
'in' means one lot of letters goes inside another.
'church' becomes 'ce' (abbreviation for Church of England).
'london' with letters rearranged gives 'olonnd'.
'a' put into 'olonnd' is 'olonnad'.
'olonnad' going inside 'ce' is 'COLONNADE'.
(Other definitions for colonnade that I've seen before include "Collection of columns" , "Arcade" , "Row of evenly spaced pillars" , "Row of columns supporting a roof" , "supporters in a row" .)