Crossword Genius

Gaps prone to reveal what's inside the pinafore (5)

Ross

I believe the answer is:

apron

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'pinafore' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for apron that I've seen before include "Space in front of curtain" , "Aircraft parking area" , "standing" , "conveyor-belt" , "Extension of stage in front of curtain" .)

I've seen this clue in The Hindu.
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