Flourish acquiring small chest (5)
I believe the answer is:
bosom
'chest' is the definition.
(thesaurus)
'flourish acquiring small' is the wordplay.
'flourish' becomes 'boom' ('boom' can be a synonym of 'flourish').
'acquiring' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'boom' enclosing 's' is 'BOSOM'.
(Other definitions for bosom that I've seen before include "A close affectionate and protective acceptance" , "heart of family" , "This friend is dear and close" , "Embrace" , "Woman's breast" .)