The most common and most likely answer to this crossword clue is the 13 letter word SPORTSMANSHIP
I've seen this crossword clue in The Sunday Telegraph
S
P
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R
T
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M
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More clues leading to the result SPORTSMANSHIP
Buddy explains!

The straight part
We know that "Fair acting in play" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fair acting in play = sportsmanship.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 SPORTSMANSHIP mean?
SPORTSMANSHIP noun- The behaviour exhibited in playing sports, either good or bad.
- The good attitude/behaviour displayed by players of a game; fairness, determination, winning or losing gracefully.