Daughter kept so untidy a work surface (7)
I believe the answer is:
desktop
'a work surface' is the definition.
(the top of one's desk)
'daughter kept so untidy' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'untidy' is an anagram indicator.
'kept'+'so'='keptso'
'keptso' anagrammed gives 'esktop'.
'd'+'esktop'='DESKTOP'
(Other definitions for desktop that I've seen before include "working area in office" , "Sort of publishing" , "Publishers'" , "Like a computer system" , "often supports a mass of papers?" .)