Being arrested, knight became ill (6)
I believe the answer is:
'being arrested' is the definition.
(I've seen this before)
'knight became ill' is the wordplay.
'knight' becomes 'N' (abbreviation for knight in chess).
'became ill' becomes 'ailed' (ail can mean to sicken).
(Other definitions for nailed that I've seen before include "Accomplished", "Fastened with metal pin", "Fastened with metal pins", "Caught, collared", "Firmly attached -- arrested".)