Case made by conservative proposal (6)
I believe the answer is:
coffer
'case' is the definition.
(thesaurus)
'by conservative proposal' is the wordplay.
'by' says to put letters next to each other.
'conservative' becomes 'C' (UK political abbreviation).
'proposal' becomes 'offer' (an offer is a proposal or suggestion).
'c' put next to 'offer' is 'COFFER'.
'made' acts as a link.
(Other definitions for coffer that I've seen before include "Treasury - chest for valuables" , "Strong box at the treasury" , "jewellery store?" , "safe" , "Small chest for keeping valuables" .)