The most common and most likely answer to this crossword clue is the 7 plus 7 letter word PYRRHIC VICTORY
I've seen this crossword clue in The Guardian
P
Y
R
R
H
I
C
V
I
C
T
O
R
Y
Buddy explains!
The straight part
We know that "dubious triumph" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dubious triumph = pyrrhic victory.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.