Disheartened hippy, softly expresses disagreement with sibling concerning trance-like state (8)
I believe the answer is:
hypnosis
'state' is the definition.
(hypnosis is a kind of state)
'disheartened hippy softly expresses disagreement with sibling' is the wordplay.
'disheartened' suggests removing the centre (the middle or heart is removed).
'softly' becomes 'p' (short for piano, musical term for softly).
'expresses disagreement with' becomes 'no' (I can't justify this - if you can you should believe this answer much more).
'sibling' becomes 'sis' (short for sister).
'hippy' with its middle removed is 'hy'.
'hy'+'p'+'no'+'sis'='HYPNOSIS'
'concerning trance-like' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for hypnosis that I've seen before include "Inducing of trance-like state" , "Suggestive technique" , "Induced state of trance" , "State induced by suggestion, resembles sleep" , "Means of inducing trance" .)