One with nervous trouble can be really sour (6)
I believe the answer is:
acetic
'be really sour' is the definition.
(I've seen this before)
'one with nervous trouble' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'with nervous trouble' becomes 'tic' (I can't justify this - if you can you should believe this answer much more).
'ace'+'tic'='ACETIC'
'can' acts as a link.
Can you help me to learn more?
(Other definitions for acetic that I've seen before include "really sour" , "sort of acid" , "tart" , "Like the acid in vinegar" , "Saucy'" .)