The most common and most likely answer to this crossword clue is the 7 letter word INBREED
I've seen this crossword clue in The Guardian
I
N
B
R
E
E
D
More clues leading to the result INBREED
Buddy explains!
The straight part
We know that "Multiply without going forth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case multiply without going forth = inbreed.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 INBREED mean?
INBREED verb- To breed or reproduce with those that are related.
- To breed with those that share common traits or qualities.
- To produce or generate within.