Showing bust firm left beset by strike (9)
I believe the answer is:
decollete
'showing bust' is the definition.
'firm left beset by strike' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'left' becomes 'l' (common abbreviation).
'beset by' indicates putting letters inside ('beset' could historically mean 'cover').
'strike' becomes 'delete' (**).
'co'+'l'='col'
'col' inserted within 'delete' is 'DECOLLETE'.
(Other definitions for decollete that I've seen before include "Of clothes, low-cut" , "Low cut" , "low-cut dress!" , "exposing milk suppliers" , "Having a low neckline, from French" .)