Backing FBI agent with close support (6)
I believe the answer is:
defend
'support' is the definition.
(I know that support can be written as defend)
'backing fbi agent with close' is the wordplay.
'backing' is a reversal indicator.
'fbi agent' becomes 'fed' (short for federal agent).
'with' says to put letters next to each other.
'close' becomes 'end' (close can mean to end or finish).
'fed' backwards is 'def'.
'def'+'end'='DEFEND'
(Other definitions for defend that I've seen before include "Protect or assist against attack" , "Shield" , "Guard against attack" , "Compete to retain (sporting title)" , "Protect from harm" .)