Act as a substitute (4,2)
I believe the answer is:
step in
'act as a substitute' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for step in that I've seen before include "enter" , "Type of garment" , "Shoe with no fastenings" , "Intervene dramatically, or at a senior level" , "Substitute" .)