Agreement to get tough, hiding depression (9)
I believe the answer is:
indenture
'agreement' is the definition.
(indenture is a kind of agreement)
'get tough hiding depression' is the wordplay.
'get tough' becomes 'inure' (I can't explain this - if you can you should give a lot more credence to this answer).
'hiding' is an insertion indicator.
'depression' becomes 'dent' (dent is a kind of depression).
'inure' going around 'dent' is 'INDENTURE'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for indenture that I've seen before include "Sealed agreement" , "Bind to service" , "Document" , "Agreement binding apprentice to a master" , "served by a novice?" .)