The most common and most likely answer to this crossword clue is the 5 plus 7 plus 3 letter word WORLD WITHOUT END
I've seen this crossword clue in The Times
W
O
R
L
D
W
I
T
H
O
U
T
E
N
D
More clues leading to the result WORLD WITHOUT END
Buddy explains!
The straight part
We know that "Infinity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case infinity = world without end.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.