Dawn French very hot inside reserve (9)
I believe the answer is:
threshold
'dawn' is the definition.
(both can mean a beginning or opening)
'french very hot inside reserve' is the wordplay.
'french very' becomes 'tres' ('very' translated into French).
'hot' becomes 'h'.
'inside' means one lot of letters goes inside another.
'reserve' becomes 'hold' ('hold' can be a synonym of 'reserve'**).
'tres' placed around 'h' is 'thres'.
'thres'+'hold'='THRESHOLD'
(Other definitions for threshold that I've seen before include "Verge" , "Point of entry" , "Doorway, entrance" , "Entrance to house - bride is carried across it" , "Beginning" .)