Be audacious in the fitting-room? (3,2,2)
I believe the answer is:
try it on
'be audacious' is the definition.
(I've seen this before)
'in the fitting-room?' is the wordplay.
'in' indicates putting letters inside.
'the fitting' becomes 'try-on' ('try-on' can be a synonym of 'fitting').
'room?' becomes 'it' (I can't explain this - if you can you should believe this answer much more).
'tryon' going around 'it' is 'TRY IT ON'.
(Other definitions for try it on that I've seen before include "Attempt to fool" , "Before buying a dress, you should" , "Deliberately test someone's patience (colloq.)" , "practise a little deception" , "Be audacious" .)