Delicious introduction to yeasty ... er ... goodness (5)
I believe the answer is:
yummy
'delicious' is the definition.
('yummy' can be a synonym of 'delicious')
'introduction to yeasty er goodness' is the wordplay.
'introduction to' indicates taking the first letters (I've seen 'introductions to' mean this).
'er' becomes 'um'.
'goodness' becomes 'my' (both are exclamations of surprise).
The first letter of 'yeasty' is 'y'.
'y'+'um'+'my'='YUMMY'
(Other definitions for yummy that I've seen before include "Like delicious food" , "That tastes good!" , "Mmmmm!" , "Very tasty" , "Mmmmmm!" .)