Fixing the south-east extension (6)
I believe the answer is:
sewing
'fixing' is the definition.
(sewing is a kind of fixing)
'south-east extension' is the wordplay.
'south east' becomes 'SE' (abbreviation).
'extension' becomes 'wing' (synonyms).
'se'+'wing'='SEWING'
'the' is the link.
(Other definitions for sewing that I've seen before include "Darning" , "Working with a needle" , "Genteel pursuit" , "Needlework" , "Using a needle and thread" .)