The most common and most likely answer to this crossword clue is the 10 letter word TIMESERVER
I've seen this crossword clue in The Daily Telegraph
T
I
M
E
S
E
R
V
E
R
More clues leading to the result TIMESERVER
Buddy explains!

The straight part
We know that "Opportunist" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case opportunist = timeserver.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 TIMESERVER mean?
TIMESERVER noun- Someone who honours their commitments only when it is personally easy to do so.
- A person who conforms to current opinions, especially for reasons of personal advantage; an opportunist.
- Someone who performs a job for the required time only, making a minimum of effort.
- A device, node or program that transmits the correct time to clients in a network.