Relentless; persistent
I believe the answer is:
unabated
I believe this clue is a double definition.
'relentless' is the first definition.
(thesaurus)
'persistent' is the second definition.
(I've seen this before)
(Other definitions for unabated that I've seen before include "Undiminished" , "with no loss of intensity" , "Without diminution" , "Persistent" , "as strong as ever" .)