Oppressively constant (10)
I believe the answer is:
relentless
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">oppressively constant</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for relentless that I've seen before include "Persistent" , "Uncompromising" , "Harsh, inflexible" , "Oppressively constant; inflexible" , "Allowing no respite" .)