The most common and most likely answer to this crossword clue is the 7 letter word T-SHIRT
T
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S
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T
Buddy explains!

The straight part
We know that "Top" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case top = t-shirt.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 T-SHIRT mean?
T-SHIRT noun- A lightweight shirt without buttons, usually with short sleeves and no collar. Often made of cotton and frequently bears a picture or slogan.
- An undershirt.