Single-minded and certain to hold volunteers back (9)
I believe the answer is:
dedicated
'single-minded' is the definition.
(I've seen this before)
'certain to hold volunteers back' is the wordplay.
'certain' becomes 'decided'.
'to hold' is an insertion indicator.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'back' says the letters should be written backwards.
'decided' going around 'ta' is 'detacided'.
'detacided' in reverse letter order is 'DEDICATED'.
'and' acts as a link.
(Other definitions for dedicated that I've seen before include "Devoted to a particular cause or purpose" , "Hard working" , "exclusive" , "Customised" , "Devoted to a task or purpose" .)