What’s served at breakfast for one guest at the outset (3)
I believe the answer is:
egg
'what's served at breakfast' is the definition.
(something eaten at breakfast)
'for one guest at the outset' is the wordplay.
'for one' becomes 'eg' (both can mean 'for example').
'at the outset' indicates taking the first letters.
The first letter of 'guest' is 'g'.
'eg'+'g'='EGG'
(Other definitions for egg that I've seen before include "nit? Just a bit!" , "Hen fruit" , "Baby carrier?" , "Laid item" , "On the face, its an embarrassment" .)