Compact mountain group (6)
I believe the answer is:
massif
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">compact mountain group</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 massif that I've seen before include "Geological formation of an IMF ass" , "Mountainous block" , "huge piece of crust" , "IMF ass found on mountainous plateau" , "French mountain range" .)