The most common and most likely answer to this crossword clue is the 6 letter word EQUALS
E
Q
U
A
L
S
Buddy explains!

The straight part
We know that "Measures up/rivals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case measures up/rivals = equals.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 EQUALS mean?
EQUALS noun- A person or thing of equal status to others.
- State of being equal; equality.
- To be equal to, to have the same value as; to correspond to.
- To make equivalent to; to cause to match.
- To have as its consequence.
- The symbol =