An opening filled by daughter in period before Christmas (6)
I believe the answer is:
advent
'period before christmas' is the definition.
(advent is the period leading up to Christmas Day)
'an opening filled by daughter' is the wordplay.
'an' becomes 'a'.
'opening' becomes 'vent' (vent is a kind of opening).
'filled by' indicates putting letters inside.
'daughter' becomes 'd' (genealogical abbreviation).
'a'+'vent'='avent'
'avent' placed around 'd' is 'ADVENT'.
'in' acts as a link.
(Other definitions for advent that I've seen before include "Period in church calendar" , "Period in the Christian calendar" , "Pre-Christmas season for Christians" , "Time approaching Christmas" , "The season leading up to Christmas" .)