The most common and most likely answer to this crossword clue is the 2 plus 2 letter word SO SO
I've seen this crossword clue in Independent
S
O
S
O
More clues leading to the result SO SO
Buddy explains!

The straight part
We know that "just about adequate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case just about adequate = so so.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 SO SO mean?
SO SO adjective- Neither good nor bad; tolerable, passable, indifferent.
- Neither very well nor very poorly.