Strongboxes commandos lined with iron (5)

I believe the answer is:

safes

'strongboxes' is the definition.
(safe is a kind of strongbox)

'commandos lined with iron' is the wordplay.
'commandos' becomes 'sas' (Special Air Service).
'lined with' means one lot of letters goes inside another (I've seen this in other clues).
'iron' becomes 'fe' ('Fe' can be a synonym of 'iron').
'sas' going around 'fe' is 'SAFES'.

I've seen this clue in The Times.

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