Compact group of mountains (6)
I believe the answer is:
massif
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">compact group of mountains</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 "High points" , "Highland area?" , "Block of mountains" , "huge piece of crust" , "Compact mountainous area" .)