Not varying
I believe the answer is:
uniform
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not varying</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 uniform that I've seen before include "Consistent, unvarying" , "reluctant pupils may be forced to wear it" , "Same - suit" , "Constant; outfit" , "Unvarying kind of costume" .)