Thus a heavy blow comes earlier (6)
I believe the answer is:
sooner
'earlier' is the definition.
(synonyms)
'thus a heavy blow' is the wordplay.
'thus' becomes 'so' (synonyms).
'a heavy blow' becomes 'oner' (I've seen this before).
'so'+'oner'='SOONER'
'comes' acts as a link.
(Other definitions for sooner that I've seen before include "native of Oklahoma" , "nickname of US State" , "Earlier than previously thought" , "Rather" , "Earlier or preferably" .)