Knight wasn't well attached somehow (6)
I believe the answer is:
nailed
'attached somehow' is the definition.
(attached with a nail)
'knight wasn't well' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'wasn't well' becomes 'ailed' (to ail is to be ill).
'n'+'ailed'='NAILED'
(Other definitions for nailed that I've seen before include "Hammered in" , "Fastened by Daniel" , "Caught, collared" , "Properly fixed" , "Fixed with a pin" .)