King, say, is keeping in mind what soldiers should wear (11)

I believe the answer is:

regimentals

'what soldiers should wear' is the definition.

'king say is keeping in mind' is the wordplay.
'king say' becomes 'regis' (Latin for 'of the king').
'is keeping' indicates putting letters inside.
'in mind' becomes 'mental'.
'regis' enclosing 'mental' is 'REGIMENTALS'.

I've seen this clue in The Telegraph.

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