Dissolute young trainee mostly overwhelmed with depression (8)
I believe the answer is:
decadent
'dissolute' is the definition.
(I've seen this before)
'young trainee mostly overwhelmed with depression' is the wordplay.
'young trainee' becomes 'cadet' (eg an army cadet).
'mostly' means to remove the last letter.
'overwhelmed with' means one lot of letters goes inside another (I've seen this in other clues).
'depression' becomes 'dent' (dent is a kind of depression).
'cadet' with its final letter taken away is 'cade'.
'cade' going into 'dent' is 'DECADENT'.
(Other definitions for decadent that I've seen before include "Dissolute and corrupt" , "Depraved" , "Morally corrupt" , "Abandoning principles" , "Decanted in corrupt way" .)