Bust — or its opposite, hiding beginning of slump (5)
I believe the answer is:
bosom
'bust' is the definition.
(I've seen this before)
'opposite hiding beginning of slump' is the wordplay.
'opposite' becomes 'boom' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'hiding' means one lot of letters goes inside another.
'beginning of' says to take the initial letters.
The initial letter of 'slump' is 's'.
'boom' placed around 's' is 'BOSOM'.
'or its' acts as a link.
Can you help me to learn more?
(Other definitions for bosom that I've seen before include "Woman's chest" , "Woman's breast" , "Female breasts" , "heart of family" , "Breast, chest" .)