A tall one may be difficult to carry out (5)

I believe the answer is:

order

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'carry out' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?

I don't understand the rest of the clue.

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(Other definitions for order that I've seen before include "Decree" , "Reserved goods" , "Command, mandate" , "Rank" , "Instruct" .)

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

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