Growing tissue closed within substitute moon craft (10)

I believe the answer is:

protoxylem

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'growing tissue' is the definition.
The definition suggests a singular noun which matches the answer.

'closed within substitute moon craft' is the wordplay.
'closed' becomes 'to' (as in a door closed to).
'within' indicates putting letters inside.
'substitute' becomes 'proxy' (I've seen this in another clue).
'moon craft' becomes 'lem' (I've seen this in another clue).
'to' put within 'proxy' is 'protoxy'.
'protoxy'+'lem'='PROTOXYLEM'

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I've seen this clue in The Telegraph.

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