If expressed in OCaml, it's just

let compose f5 f4 f3 f2 f1 f0 x0 x1 x2 = f5 (f4 (f3 (f2 (f1 (f0 x0 x1 x2)))))

whose signature is,

val compose : ('a -> 'b) -> ('c -> 'a) -> ('d -> 'c) -> ('e -> 'd) -> ('f -> 'e) -> ('g -> 'h -> 'i -> 'f) -> 'g -> 'h -> 'i -> 'b = <fun>