Regroup behind mountains (9)

I believe the answer is:

rearrange

'regroup' is the definition.
(to arrange or group differently)

'behind mountains' is the wordplay.
'behind' becomes 'rear' (synonyms).
'mountains' becomes 'range' (I've seen this before).
'rear'+'range'='REARRANGE'

(Other definitions for rearrange that I've seen before include "Change the position of" , "Put new structure" , "Tidy up" , "Rear anger (anag.)" , "Move round" .)

I've seen this clue in The Guardian.

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