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' means one lot of letters goes inside another (some letters go outside others).
'so' placed into 'man' is 'MASON'.
'is a' is the 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" .)