Was a substitute informed? (6,2)
I believe the answer is:
filled in
I believe this clue is a double definition.
'was a substitute' is the first definition.
(I've seen this before)
'informed?' is the second definition.
(filling in is a kind of informing)
(Other definitions for filled in that I've seen before include "well-informed" , "Acted as substitute" , "informed?" .)