Colleague's going west, skirting constant danger to shipping (6)
I believe the answer is:
Here is my best explanation:
'danger to shipping' is the definition.
(I've seen this before)
'colleague's going west skirting constant' is the subsidiary indication.
'colleague's' becomes 'allys' (I can't justify this - if you can you should believe this answer much more).
'going west' says the letters should be written backwards.
'skirting' indicates putting letters inside.
'constant' becomes 'c' (mathematical abbreviation).
'allys' backwards is 'sylla'.
'sylla' placed around 'c' is 'scylla'.
(Other definitions for scylla that I've seen before include "Six-headed monster who sat on a rock opposite Charybdis", "Charybdis counterpart", "legendary rock", "danger to shipping", "Monster", "Artificial cave", "Counterpart to Charybdis", "Mythic sea nymph transformed into a sea monster who devoured sailors", "Peril with Charybdis" and "Rock, across from 21".)