The most common and most likely answer to this crossword clue is the 8 letter word GAMECOCK
I've seen this crossword clue in The Guardian
G
A
M
E
C
O
C
K
Buddy explains!

The straight part
We know that "trained fighter /trained fighter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trained fighter /trained fighter = gamecock.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 GAMECOCK mean?
GAMECOCK noun- A fighting cock: a rooster used in cockfighting.