About fifty, overweight and dull (4)
I believe the answer is:
flat
'dull' is the definition.
('flat' can be a synonym of 'dull')
'about fifty overweight' is the wordplay.
'about' means one lot of letters goes inside another.
'fifty' becomes 'l' (roman numeral).
'overweight' becomes 'fat' (I've seen this before).
'l' going within 'fat' is 'FLAT'.
'and' acts as a link.
(Other definitions for flat that I've seen before include "Small residence - lacking sparkle" , "like some races" , "Punctured tyre" , "Level; dull" , "Dull; apartment" .)