’oists part of the roof (5)
I believe the answer is:
eaves
This clue is a double definition.
''oists' is the first definition.
('heaves' - I've seen this before - said with a cockney accent, dropping the 'H', like 'hoists' was)
'part of the roof' is the second definition.
('eaves' is part of 'roof')
(Other definitions for eaves that I've seen before include "what might constitute a hangover?" , "Overhanging part of roof" , "Overhang at edge of a roof" , "Projecting edge of a roof" , "Roof's overhang" .)