The most common and most likely answer to this crossword clue is the 6 letter word UPHOLD
I've seen this crossword clue in The Daily Telegraph
U
P
H
O
L
D
More clues leading to the result UPHOLD
Buddy explains!

The straight part
We know that "Maintain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case maintain = uphold.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 UPHOLD mean?
UPHOLD verb- To hold up; to lift on high; to elevate.
- To keep erect; to support; to sustain; to keep from falling
- To support by approval or encouragement, to confirm (something which has been questioned)