Gaps prone to reveal what's inside the pinafore (5)
I believe the answer is:
apron
'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" .)