The most common and most likely answer to this crossword clue is the 4 letter word FAIN
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More clues leading to the result FAIN
Buddy explains!

The straight part
We know that "Glad" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case glad = fain.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 FAIN mean?
FAIN adjective- Well-pleased, glad.
- Satisfied, contented.
- Eager, willing or inclined to.
- Obliged or compelled to.
- With joy; gladly.
- By will or choice.
- To be delighted or glad; to rejoice.
- To gladden.