Before taking power, grow one root crop (6)

I believe the answer is:

turnip

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'root crop' is the definition.

'before taking power grow one' is the wordplay.
'before' means one lot of letters go next to another.
'taking power' becomes 'p' (symbol in physics. I am not sure about the 'taking' bit.).
'grow' becomes 'turn' (synonyms).
'one' becomes 'i' (Roman numeral).
'turn'+'i'='turni'
'p' after 'turni' is 'TURNIP'.

Can you help me to learn more?

(Other definitions for turnip that I've seen before include "'Root vegetable, yellow or white (6)'" , "Watch" , "veggie" , "Fleshy edible white or yellow root" , "Rip nut up for a vegetable" .)

I've seen this clue in The Times.

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