So the fellow outside is a builder (5)
I believe the answer is:
mason
'builder' is the definition.
(I've seen this before)
'so the fellow outside' is the wordplay.
'the fellow' becomes 'man' (fellow can mean a man).
'outside' indicates putting letters inside (some letters go outside others).
'so' going within 'man' is 'MASON'.
'is a' acts as a link.
(Other definitions for mason that I've seen before include "One is skilful with stone" , "Craftsperson who works with stone or brick" , "A skilled worker in stone" , "figure in lodge" , "one in construction" .)