Top of wall is good fit (5)
I believe the answer is:
gable
'top of wall' is the definition.
(architectural term)
'good fit' is the wordplay.
'good' becomes 'g' (abbreviation).
'fit' becomes 'able' (similar in meaning).
'g'+'able'='GABLE'
'is' acts as a link.
(Other definitions for gable that I've seen before include "screen legend" , "Triangular brickwork" , "Part of house" , "Section of wall between roof slopes" , "Triangular part of a wall supporting a ridged roof" .)