Exhibiting bust key, holding pass (9)
I believe the answer is:
decollete
'exhibiting bust' is the definition.
(decollete means having a low neckline)
'key holding pass' is the wordplay.
'key' becomes 'delete' (key on a computer keyboard).
'holding' is an insertion indicator.
'pass' becomes 'col' (col is a term for a mountain pass).
'delete' enclosing 'col' is 'DECOLLETE'.
(Other definitions for decollete that I've seen before include "Low cut" , "exposing milk suppliers" , "Having a low neckline, from French" , "Of clothes, low-cut" , "low-cut dress!" .)