Colleague's going west, skirting constant danger to shipping (6)
I believe the answer is:
scylla
'danger to shipping' is the definition.
(I've seen this before)
'colleague's going west skirting constant' is the wordplay.
'colleague's' becomes 'allys'.
'going west' says the letters should be written in reverse.
'skirting' indicates putting letters inside.
'constant' becomes 'c' (mathematical abbreviation).
'allys' in reverse letter order is 'sylla'.
'sylla' placed around 'c' is 'SCYLLA'.
(Other definitions for scylla that I've seen before include "Legendary rock" , "Rock, across from 21" , "Mythic sea nymph transformed into a sea monster who devoured sailors" , "Six-headed monster who sat on a rock opposite Charybdis" , "Mythical rock" .)