The most common and most likely answer to this crossword clue is the 6 letter word FIXATE
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Buddy explains!

The straight part
We know that "Closely focus on" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case closely focus on = fixate.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 FIXATE mean?
FIXATE verb- To make something fixed and stable; to fix.
- To stare fixedly at something.
- To attend to something to the exclusion of all others; used with on.
- To attach oneself to a person or thing in a pathological or neurotic manner; used with on.