Way to include constant repair (5)
I believe the answer is:
patch
'repair' is the definition.
(I know that patch is a more specific form of the action repair)
'way to include constant' is the wordplay.
'way' becomes 'path' (synonyms).
'to include' indicates putting letters inside.
'constant' becomes 'c' (common symbol in maths for an unknown constant).
'path' enclosing 'c' is 'PATCH'.
(Other definitions for patch that I've seen before include "Mend with piece of material" , "Material used to cover a tear" , "Piece of land, of mending material" , "Mend by adding fabric" , "temporary repair" .)