Mountain group (6)
I believe the answer is:
massif
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mountain group</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for massif that I've seen before include "Mountain centre" , "mountain peak" , "Mountain block" , "French mountain range" , "Geographical formation" .)