The most common and most likely answer to this crossword clue is the 8 letter word REDOUBLE
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More clues leading to the result REDOUBLE
Buddy explains!

The straight part
We know that "Increase" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case increase = redouble.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 REDOUBLE mean?
REDOUBLE noun- An optional bid made by the side currently holding the highest bid for the contract, after the opposing side has doubled.
- To double, especially to double again; to increase considerably; to multiply; to intensify.
- To double an opponent's doubling bid.
- To become twice as big.
- To double again what was reduced to a single state.