Group of women, quiet, mostly packed before slim monarch reveals unrealistic belief (7,8)
I believe the answer is:
wishful thinking
'unrealistic belief' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'group of women quiet mostly packed before slim monarch' is the wordplay.
'group of women' becomes 'WI' (Women's Institute).
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'mostly' means to remove the last letter (most but not all of the word).
'packed' becomes 'full' (synonyms).
'before slim monarch' becomes 'thinking' (I can't explain this - if you can you should give a lot more credence to this answer).
'full' with its last letter taken away is 'ful'.
'wi'+'sh'+'ful'+'thinking'='WISHFUL THINKING'
'reveals' acts as a link.
Can you help me to learn more?
(Other definitions for wishful thinking that I've seen before include "pure fantasy" , "over-optimistic expectation" .)