The most common and most likely answer to this crossword clue is the 10 letter word PLUPERFECT
I've seen this crossword clue in Independent
P
L
U
P
E
R
F
E
C
T
Buddy explains!

The straight part
We know that "tense" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tense = pluperfect.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 PLUPERFECT mean?
PLUPERFECT adjective- Being a generalization of a perfect number. For a given natural number k, a number n is called k-perfect (or k-fold perfect) iff the sum of all positive divisors of n (the divisor function, σ(n)) is equal to kn.
- The pluperfect tense.
- A verb in this tense.
- More than perfect.
- (grammar) Pertaining to action completed before or at the same time as another.
- Relating to a certain type of graph, complying with the theorem (pluperfect graph theorem) discovered by D. R. Fulkerson in 1970.
- Used as an intensifier in various interjections.