Mountaineer (7)
I believe the answer is:
climber
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mountaineer</span>' is the definition.<br/ ><span class="explanation-format__explanation">(someone climbing mountains)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for climber that I've seen before include "Plant that grows up a support" , "Bonnington" , "Edward Whymper, for example" , "Mountaineer" , "in the garden" .)