Trenches so safe when there's been an upheaval (6)

I believe the answer is:

fossae

'trenches' is the definition.
(plural of fossa - a fossa is a depression)

'so safe when there's been an upheaval' is the wordplay.
'when there's been an upheaval' indicates an anagram (letters go through change or upheaval).
'so'+'safe'='sosafe'
'sosafe' with letters rearranged gives 'FOSSAE'.

(Another definition for fossae that I've seen is " pits".)

I've seen this clue in The Guardian.

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