One about to shelter under a dry space in building (5)
I believe the answer is:
attic
'space in building' is the definition.
'one about to shelter under a dry' is the wordplay.
'one about' becomes 'ic' (I've seen this in other clues**).
'to shelter under' says to put letters next to each other.
'dry' becomes 'tt' (abbreviation for teetotal).
'a'+'tt'='att'
'ic' put after 'att' is 'ATTIC'.
(Other definitions for attic that I've seen before include "Ancient Greek; loft" , "Top chamber" , "High level of" , "from Greece" , "top-flight accommodation" .)