Soldiers shagged in bed? (7)

I believe the answer is:

retired

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'in bed?' is the definition.
(I've seen this before)

'soldiers shagged' is the wordplay.
'soldiers' becomes 'RE' (Royal Engineers).
'shagged' becomes 'tired' (I can't justify this - if you can you should believe this answer much more).
're'+'tired'='RETIRED'

Can you help me to learn more?

(Other definitions for retired that I've seen before include "out of office?" , "Ceased working" , "claiming a pension?" , "Stopped work - went to bed" , "Stopped working" .)

I've seen this clue in The Independent.

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