Leading shareholder in boom and bust (5)
I believe the answer is:
bosom
'bust' is the definition.
(I've seen this in another clue)
'leading shareholder in boom' is the wordplay.
'leading' indicates taking the first letters.
'in' indicates putting letters inside.
The initial letter of 'shareholder' is 's'.
's' inserted into 'boom' is 'BOSOM'.
'and' acts as a link.
(Other definitions for bosom that I've seen before include "Female breasts" , "Breast, chest" , "5 [BUST]" , "Intimate" , "A close affectionate and protective acceptance" .)