The most common and most likely answer to this crossword clue is the 5 letter word GONAD
G
O
N
A
D
Buddy explains!
The straight part
We know that "plays a part in reproduction" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plays a part in reproduction = gonad.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 GONAD mean?
GONAD noun- A sex organ that produces gametes; specifically, a testicle or ovary.
- (chiefly in the plural) The testicles.