Every other layperson and knight on board the Pinafore (5)
I believe the answer is:
apron
'the pinafore' is the definition.
(I've seen this in another clue)
'every other layperson and knight on board' is the wordplay.
'every other' means one should take alternating letters.
'and' means one lot of letters go next to another.
'knight on board' becomes 'N' (chess abbreviation for knight).
The alternate letters of 'layperson' are 'apro'.
'apro'+'n'='APRON'
(Other definitions for apron that I've seen before include "Protective garment - type of stage" , "item of clothing in the kitchen" , "Part of stage, boxing ring or airport terminal" , "Extension of stage in front of curtain" , "Kitchen garment or part of airfield" .)