The most common and most likely answer to this crossword clue is the 6 plus 4 letter word SECOND RATE
I've seen this crossword clue in The Daily Telegraph
S
E
C
O
N
D
R
A
T
E
More clues leading to the result SECOND RATE
Buddy explains!
The straight part
We know that "Mediocre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mediocre = second rate.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 SECOND RATE mean?
SECOND RATE noun- A British ship-of-the-line sail warship having 90 to 98 guns dispersed on three gundecks.
- Of mediocre quality; not first class.