The most common and most likely answer to this crossword clue is the 4 letter word THAN
T
H
A
N
More clues leading to the result THAN
Buddy explains!

The straight part
We know that "used to indicate comparison/it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case used to indicate comparison/it = than.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 THAN mean?
THAN adverb- At that time; then.
- Introduces a comparison, and is associated with comparatives, and with words such as more, less, and fewer. Typically, it seeks to measure the force of an adjective or similar description between two predicates.
- Used in comparisons, to introduce the basis of comparison.
- (obsolete outside dialectal, usually used with for) Because; for.