Wise old head active in emergencies (8,5)
I believe the answer is:
eminence grise
'wise old head' is the definition.
(French term for a wise or influential person)
'active in emergencies' is the wordplay.
'active' indicates anagramming the letters (the letters are active/moving).
'in'+'emergencies'='inemergencies'
'inemergencies' anagrammed gives 'EMINENCE GRISE'.
(Other definitions for eminence grise that I've seen before include "Power behind the throne" , "One exercising power in the background" .)