Single-minded reactionary resolved to protect old volunteers (9)
I believe the answer is:
dedicated
'single-minded' is the definition.
(I've seen this before)
'reactionary resolved to protect old volunteers' is the wordplay.
'reactionary' shows that the letters should be reversed in order (reactions happen in the opposite direction).
'resolved' becomes 'decided' ('decide' can be a synonym of 'resolve').
'to protect' means one lot of letters goes inside another (inserted letters are held or protected).
'old volunteers' becomes 'TA' (Territorial Army, former name of the Army Reserve).
'decided' going around 'ta' is 'detacided'.
'detacided' backwards is 'DEDICATED'.
(Other definitions for dedicated that I've seen before include "Diligent" , "Customised" , "Determined" , "Hard working" , "Devoted to a particular cause or purpose" .)