Crazy act in field saves old knight and private (12)
I believe the answer is:
confidential
'private' is the definition.
(similar in meaning)
'crazy act in field saves old knight' is the wordplay.
'crazy' is an anagram indicator.
'saves' is an insertion indicator.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'knight' becomes 'n' (chess abbreviation).
'act'+'in'+'field'='actinfield'
'actinfield' with letters rearranged gives 'cfidential'.
'o'+'n'='on'
'cfidential' enclosing 'on' is 'CONFIDENTIAL'.
'and' acts as a link.
(Other definitions for confidential that I've seen before include "not expecting publication" , "Like T.S. Eliot's clerk" , "Graham Greene's agent" , "Classified" , "Inflicted on a (anag) - private" .)