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' (I've seen this before).
'outside' indicates putting letters inside.
'so' placed within '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" .)