What to say at bedtime about following evening (9)
I believe the answer is:
nightfall
'evening' is the definition.
(thesaurus)
'what to say at bedtime about following' is the wordplay.
'what to say at bedtime' becomes 'nightall' ('night all' is a way of saying goodnight).
'about' means one lot of letters goes inside another.
'following' becomes 'f' (used when citing page numbers etc).
'nightall' going around 'f' is 'NIGHTFALL'.
(Other definitions for nightfall that I've seen before include "when visibility reduces" , "The onset of darkness" , "End of day" , "after sunset" , "when evenings draw in" .)