What windbags do best? (9)
I believe the answer is:
uttermost
'best?' is the definition.
(uttermost can mean the highest or best)
'what windbags do' is the wordplay.
I cannot quite understand how this works, but
'do' could be 'ut' (synonyms) and 'ut' is found in the answer.
This explanation may well be incorrect...
(Other definitions for uttermost that I've seen before include "Chief" , "The furthest point or degree" , "Maximum - extremity" , "at the furthest limit" , "best" .)