Staunch supporter certain at heart to regret waste (4,4)
I believe the answer is:
true blue
'staunch supporter' is the definition.
(supporter of the Conservative Party)
'certain at heart to regret waste' is the wordplay.
'at heart' means to look at the middle letters.
'to regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'waste' becomes 'blue' (blue can mean to squander or waste).
The middle letter of 'certain' is 't'.
't'+'rue'+'blue'='TRUE-BLUE'
(Other definitions for true blue that I've seen before include "Staunch Conservative" , "Very much Conservative" , "Loyally Conservative" , "Staunch Tory" , "Firmly loyal in cerulean manner?" .)