Women's group, quiet, almost packed, with slim monarch? That's an unrealistic hope (7,8)
I believe the answer is:
wishful thinking
'an unrealistic hope' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'women's group quiet almost packed with slim monarch?' is the wordplay.
'women's group' becomes 'wi' (Women's Institute).
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'almost' means to remove the last letter (most of the word but not all of it).
'packed' becomes 'full' (synonyms).
'with 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 final letter taken away is 'ful'.
'wi'+'sh'+'ful'+'thinking'='WISHFUL THINKING'
'that's' 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" .)