Demands European detective follows new eyewitness's lead (5)
I believe the answer is:
needs
'demands' is the definition.
('need' can be a synonym of 'demand')
'european detective follows new eyewitness's lead' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'detective' becomes 'DS' (detective sergeant).
'follows' says to put letters next to each other.
'lead' says to take the initial letters.
The initial letters of 'new eyewitnesss' is 'ne'.
'e'+'ds'='eds'
'eds' put after 'ne' is 'NEEDS'.
(Other definitions for needs that I've seen before include "necessity once" , "Requirements" , "Necessities" , "Wants, requires" , "Requires, is in want of" .)