27 across
Once
partner
is
following,
beginning
to
introduce
glimpse
of
billy
goat
(4)
Once partner is following, beginning to introduce glimpse of billy goat (4)
hints
0
checks
0
hints
The definition is at the end of the clue.
How the clue works
Look at goat at the end — that's your definition, since an ibex is a wild mountain goat. For the wordplay, you're building it up piece by piece: I is the "beginning to introduce", B is the "glimpse of billy" (just the first letter), and then EX is your "once partner", tagged on last as "following" tells you it comes after the rest.