Crossword Genius

Very reliable person's powerful tractor? (5,2,8)

Ross

I believe the answer is:

tower of strength

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'powerful tractor?' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

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(Other definitions for tower of strength that I've seen before include "One very reliable" , "one you can depend on" , "tugboat?" , "Person giving support" , "Powerful tractor" .)

I've seen this clue in The Telegraph.
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