In favour of wearing an overall (5)
I believe the answer is:
apron
'overall' is the definition.
'in favour of wearing an' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'wearing' means one lot of letters goes inside another.
'pro' put within 'an' is 'APRON'.
(Other definitions for apron that I've seen before include "Protective garment worn at front" , "edge" , "standing" , "Paved surface where aircraft stand when not in use" , "Cook's garment" .)