The most common and most likely answer to this crossword clue is the 6 letter word PREMIA
P
R
E
M
I
A
Buddy explains!

The straight part
We know that "bonuses only rarely" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bonuses only rarely = premia.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.
Crossword Answer definitions
PREMIA noun- A prize or award.
- Something offered at a reduced price as an inducement to buy something else.
- A bonus paid in addition to normal payments.
- The amount to be paid for an insurance policy.
- An unusually high value.
- The amount by which a security's value exceeds its face value.