The most common and most likely answer to this crossword clue is the 6 letter word ALLIED
I've seen this crossword clue in The Daily Telegraph
A
L
L
I
E
D
More clues leading to the result ALLIED
- Connected everything that is disconnected at first
- United with everyone that is dead
- Joint playing Delilah endlessly
- United improved, Solskjaer finally sacked
- Improved, knocking out first in league
- United scored wasting time
- Joint liability originally accepted by Delia for cooking
- A section of football, i.e. division in league
- Fighting together, every single one die in action
- Joint American and Labour leaders misrepresented
Buddy explains!

The straight part
We know that "in league" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in league = allied.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 ALLIED mean?
ALLIED adjective- Joined as allies.
- Related.
- To unite, or form a connection between, as between families by marriage, or between princes and states by treaty, league, or confederacy.
- To connect or form a relation between by similitude, resemblance, friendship, or love.