Posture from big gun supporter (8)

I believe the answer is:

carriage

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'posture' is the definition.
(I know that posture can be written as carriage)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for carriage that I've seen before include "Transportation" , "Railway vehicle" , "Posture; conveyance" , "Bearing - conveyance" , "Railway passenger coach" .)

I've seen this clue in The Sun.

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