One daughter in demand, certainly (6)
I believe the answer is:
indeed
'certainly' is the definition.
(both can be used to answer in the affirmative)
'one daughter in demand' is the wordplay.
'one' becomes 'i' (Roman numeral).
'daughter' becomes 'd' (genealogical abbreviation).
'in' is an insertion indicator.
'demand' becomes 'need' (synonyms).
'd' placed into 'need' is 'ndeed'.
'i'+'ndeed'='INDEED'
(Other definitions for indeed that I've seen before include "Unquestionably" , "Surely" , "''A friend in need is a friend . . . . . .''" , "even" , "Without question" .)