Fixed structure’s exterior made better (7)
I believe the answer is:
secured
'fixed' is the definition.
('secure' can be a synonym of 'fix')
'structure's exterior made better' is the wordplay.
'exterior' suggests removing the centre.
'made better' becomes 'cured' (I've seen this before).
'structure' with its middle taken out is 'se'.
'se'+'cured'='SECURED'
(Other definitions for secured that I've seen before include "Obtained" , "Seized" , "Got by special effort" , "Tethered" , "Rescued (anag.)" .)