Detailed reason nervous reaction is bitter in tone (7)
I believe the answer is:
caustic
'bitter in tone' is the definition.
'detailed reason nervous reaction' is the wordplay.
'detailed' means to remove the last letter (the word must be end-less or 'de-tailed').
'reason' becomes 'cause' (I've seen this before).
'nervous reaction' becomes 'tic' (I've seen this in other clues).
'cause' with its last letter taken off is 'caus'.
'caus'+'tic'='CAUSTIC'
'is' is the link.
(Other definitions for caustic that I've seen before include "Burning, corrosive" , "Corrosive - cutting" , "Burning - cutting" , "Severe" , "Chemically burning" .)