Diddy, Nelly & French Montana Discuss French's New Album in 2018 Studio Session

Diddy's trial has many thinking about his alleged crimes ... but, he rose to fame for his musical acumen -- and, TMZ has a clip of him in his professional element from back in the day.

We've obtained exclusive video from a December 2018 studio session where French Montana's showing his album to Diddy and singer-songwriter Nelly.

Check out the clip ... the crew's having a good time -- with Diddy whispering to French at times and providing other advice while listening to songs for the 2019 entry "Montana."

French clearly cares what Diddy thinks about the production ... specifically asking him what his two favorite songs are. Diddy replies he likes "No Stylist" and "Out of Your Mind" -- and, French says he's going to make both of them singles.

Montana did just that BTW ... though he had a whole bunch more singles from the album that he later released too.

Diddy rambles for a lot of the clip, and Nelly cracks some jokes -- but, it's clear they're both invested in telling French how to be the best artist he can be.

There's one moment at the end which might raise some eyebrows with hindsight ... where Diddy uses his fist as a metaphor for hitting hard with five big tracks on the album. Watch the clip until the end to hear it for yourself.

If you don't know ... French joined Bad Boy Records in 2012 and departed some years later, though it's unclear exactly when. He's regularly refused to answer questions about his former mentor or the federal case against him.

The first week of Diddy's federal sex trafficking trial is behind him ... with Cassie reading out numerous text and emails between them over the course of several days.

Dawn Richard started her testimony in the afternoon ... and, she's expected back on the stand next week.