Stress that is a c-coin (6)
I believe the answer is:
accent
'stress' is the definition.
(synonyms)
'a c-coin' is the wordplay.
'c-coin' becomes 'ccent' ('cent' said with a stutter like 'coin' was; cent is a kind of coin).
'a'+'ccent'='ACCENT'
'that is' acts as a link.
(Other definitions for accent that I've seen before include "Emphasis; mark on a letter" , "Underline" , "eg Acute" , "Special emphasis" , "Mark over a vowel, perhaps" .)