Demand and obtain old deed (5)
I believe the answer is:
exact
'demand and obtain' is the definition.
(I've seen this before)
'old deed' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'deed' becomes 'act' (an action).
'ex'+'act'='EXACT'
(Other definitions for exact that I've seen before include "Completely accurate" , "On the nail" , "Correct; insist on" , "Explicit" , "totally accurate" .)