Sort of square, man’s tie (5)
I believe the answer is:
truss
'tie' is the definition.
(trussing is a kind of tiing)
'sort of square man's' is the wordplay.
'sort of square' becomes 't' (a T-Square is a technical drawing instrument).
'man' becomes 'russ' (**).
't'+'russ'='TRUSS'
(Other definitions for truss that I've seen before include "Support - bundle - cluster" , "Surgical appliance" , "Type of support" , "Bind - structural framework" , "Architectural corbel" .)