Regrettably, the realist is more underhand (10)
I believe the answer is:
stealthier
'more underhand' is the definition.
('stealthy' can be a synonym of 'underhand')
'regrettably the realist' is the wordplay.
'regrettably' indicates an anagram.
'the'+'realist'='therealist'
'therealist' anagrammed gives 'STEALTHIER'.
'is' acts as a link.
(Other definitions for stealthier that I've seen before include "more secretive" , "More furtive" .)