Change trains leaving current platform? Not half! (9)

I believe the answer is:

transform

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'change' is the definition.
(I know that transform is a type of change)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for transform that I've seen before include "Change completely" , "Remodel" , "Subject to change" , "adapt" , "Make great change in appearance of" .)

I've seen this clue in The Telegraph.

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