Brief one on how to be a substitute? (4,2)
I believe the answer is:
fill in
'a substitute?' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for fill in that I've seen before include "Brief" , "Deputise - complete form" , "'Act as temporary substitute (4,2)'" , "Occupy (time)" , "point between courses" .)