Posh dance over in empty library creating air of calm?
I believe the answer is:
lullaby
'air of calm?' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'posh dance over in empty library' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'dance' becomes 'ball' (ball is a kind of dance).
'over' is a reversal indicator.
'in' is an insertion indicator.
'empty' suggests removing the centre.
'library' with its centre removed is 'ly'.
'ball' back-to-front is 'llab'.
'u'+'llab'='ullab'
'ullab' inserted inside 'ly' is 'LULLABY'.
'creating' acts as a link.
Can you help me to learn more?
(Other definitions for lullaby that I've seen before include "Soporific song" , "Gentle song to put baby to sleep" , "Soothing song for baby - Brahms wrote a good one" , "Soothing song for infants" , "composition" .)