A thing one can be in behind soldiers in action (6)

I believe the answer is:

crease

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'a thing one can be in behind' is the definition.
I can't judge whether this defines the answer.

'soldiers in action' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'in' is an insertion indicator.
'action' becomes 'case' (synonyms).
're' going into 'case' is 'CREASE'.

Can you help me to learn more?

(Other definitions for crease that I've seen before include "ruck" , "Dagger rarely" , "Fold, maybe unmeant" , "Line caused by folding or pressing, cloth or paper" , "iron this out" .)

I've seen this clue in The Sunday Times.

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