Depend upon (4,2)
(Other definitions for rely on that I've seen before include "Trust, be confident in" , "Trust in with confidence" , "Put trust in" .)
'depend upon' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rely that I've seen before include "Count" , "Have confidence or faith in" , "Depend, bank" , "Run and see" , "Bank (on someone)" .)