26 across
Friend
I’m
backing
to
invest
in
a
success
(5)
Friend I’m backing to invest in a success (5)
hints
0
checks
0
hints
The definition is at the beginning of the clue.
How the clue works
Right, so friend is the definition — AMIGO's just Spanish for a friend. Now "I'm" gets reversed for that "backing" instruction, giving you MI, and you slot that into A GO (that's "a success"), so you're inserting MI into A GO to build the whole thing up.