Outsiders to solve a battle mystery (7)
I believe the answer is:
secrecy
'mystery' is the definition.
(thesaurus)
'outsiders to solve a battle' is the wordplay.
'outsiders to' means to remove the middle letters (only letters on the outside of the word).
'a battle' becomes 'Crecy' (Crecy is an example).
'solve' with its centre taken out is 'se'.
'se'+'crecy'='SECRECY'
(Other definitions for secrecy that I've seen before include "openly, without protection of this" , "Suppression of information" , "Stealth; covertness" , "Covert activity" , "Confidentiality" .)