Very Republican line brings no joy (6)
I believe the answer is:
sorrow
'no joy' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'very republican line' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'republican' becomes 'R' (abbreviation in US politics).
'line' becomes 'row' (a row or line of things).
'so'+'r'+'row'='SORROW'
'brings' acts as a link.
Can you help me to learn more?
(Other definitions for sorrow that I've seen before include "Woe" , "Expression of grief" , "'One magpie for ..., two for joy (6)'" , "Deep sadness" , "Shakespeare: ''Parting is such sweet . . . . . .'' (6)" .)