The most common and most likely answer to this crossword clue is the 12 letter word IMPREGNATING
I've seen this crossword clue in Financial Times
I
M
P
R
E
G
N
A
T
I
N
G
Buddy explains!
The straight part
We know that "Putting a bun in the oven" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case putting a bun in the oven = impregnating.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does IMPREGNATING mean?
IMPREGNATING verb- To cause to become pregnant.
- To fertilize.
- To saturate, or infuse.
- To fill pores or spaces with a substance.
- To become pregnant.