The most common and most likely answer to this crossword clue is the 12 letter word PROFESSIONAL
I've seen this crossword clue in The Daily Telegraph
P
R
O
F
E
S
S
I
O
N
A
L
Buddy explains!

The straight part
We know that "Trained perso" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trained perso = professional.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 PROFESSIONAL mean?
PROFESSIONAL noun- A person who belongs to a profession
- A person who earns their living from a specified activity
- A reputation known by name
- An expert.
- Of, pertaining to, or in accordance with the (usually high) standards of a profession.
- That is carried out for money, especially as a livelihood.
- (by extension) Expert.