The most common and most likely answer to this crossword clue is the 5 letter word AGIST
I've seen this crossword clue in Independent
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Buddy explains!

The straight part
We know that "against OAPs?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case against oaps? = agist.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 AGIST mean?
AGIST verb- To take to graze or pasture, at a certain sum; used originally of the feeding of cattle in the king's forests, and collecting the money for the same.
- To charge lands etc. with any public burden.