Concerns trees, about a thousand (5)

I believe the answer is:

firms

'concerns' is the definition.
(firm is a kind of concern)

'trees about a thousand' is the wordplay.
'trees' becomes 'firs' (fir is a kind of tree).
'about' means one lot of letters goes inside another.
'a thousand' becomes 'm' ('M' can be a synonym of 'thousand').
'firs' enclosing 'm' is 'FIRMS'.

(Another definition for firms that I've seen is " Companies".)

I've seen this clue in The Globe and Mail and The Telegraph.

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