Complete brief for substitute (4,2)
I believe the answer is:
fill in
This clue is a triple definition!
'complete' is the first definition.
(as in completing a questionnaire)
'brief' is the second definition.
(as in filling someone in, giving them information)
'for substitute' is the third definition.
(synonyms)
(Other definitions for fill in that I've seen before include "'Act as temporary substitute (4,2)'" , "point between courses" , "Occupy (time)" , "Brief" , "Deputise - complete form" .)