The most common and most likely answer to this crossword clue is the 4 plus 6 letter word REAL TENNIS
I've seen this crossword clue in The Times
R
E
A
L
T
E
N
N
I
S
More clues leading to the result REAL TENNIS
- New name in development of earliest indoor sport
- Sport later modified somewhere in Ireland
- Actual score halved — wrong returns in game
- Sport elevated one in error, accepting dodgy tale
- In which one can winningly play to the gallery
- Court activity, developing in earnest, ensnaring Earl at last
- Actual number above son in climbing game
- Tears linen, playing old game
- Tight linear nets? They sag slightly here
- Sport concerning key Olympic champion
Buddy explains!
The straight part
We know that "indoor sport" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case indoor sport = real tennis.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.