Specify erratic side agent (9)
I believe the answer is:
designate
'specify' is the definition.
(I know that specify can be written as designate)
'erratic side agent' is the wordplay.
'erratic' is an anagram indicator.
'side'+'agent'='sideagent'
'sideagent' with letters rearranged gives 'DESIGNATE'.
(Other definitions for designate that I've seen before include "Identify" , "Appointed but not yet in post" , "Assign" , "Appoint" , "Allocate" .)