Secrecy ultimately maintained, is able to ’arbour killer (7)
I believe the answer is:
cyanide
'killer' is the definition.
(cyanide is a lethal poison)
'secrecy ultimately maintained is able to 'arbour' is the wordplay.
'ultimately' says to take the final letters.
'maintained' indicates putting letters inside (inserted letters get controlled or maintained).
'is able to' becomes 'can' (if you can do something you are able to do it).
''arbour' becomes ''ide' ('hide' - I've seen this before - said with the 'H' dropped, a cockney accent, like 'harbour' was).
The last letter of 'secrecy' is 'y'.
'y' placed into 'can' is 'cyan'.
'cyan'+'ide'='CYANIDE'
(Other definitions for cyanide that I've seen before include "killer" , "Extremely poisonous salt" , "Highly toxic substance" , "Nice day (anag) - poison" , "Poisonous compound" .)