A mostly dreary time — but not for children (5)
I believe the answer is:
adult
'not for children' is the definition.
(I've seen this in another clue)
'a mostly dreary time but' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'dreary' becomes 'dull' (similar in meaning).
'time but' becomes 't' (abbreviation. I am not sure about the 'but' bit.).
'dull' with its last letter taken off is 'dul'.
'a'+'dul'+'t'='ADULT'
Can you help me to learn more?
(Other definitions for adult that I've seen before include "Indecent" , "Supposedly "responsible" person" , "used to describe dirty movies" , "One no longer young" , "Full-grown" .)