Repair a big town? What falsehood (9)
I believe the answer is:
mendacity
'falsehood' is the definition.
(thesaurus)
'repair a big town?' is the wordplay.
'repair' becomes 'mend' (synonyms).
'big town?' becomes 'city' (I've seen this before).
'mend'+'a'+'city'='MENDACITY'
'what' is the link.
(Other definitions for mendacity that I've seen before include "Lying" , "Untruthfulness" , "Quality or habit of telling lies or being deceitful" .)