Supervise remaining witness (7)
I believe the answer is:
oversee
'supervise' is the definition.
(I know that supervise can be written as oversee)
'remaining witness' is the wordplay.
'remaining' becomes 'over' (as in 'we ate them all but had one over').
'witness' becomes 'see' (synonyms).
'over'+'see'='OVERSEE'
(Other definitions for oversee that I've seen before include "Direct" , "Supervise, keep an eye on" , "Administer" , "Watch people toil" , "What superintendent will do" .)