The most common and most likely answer to this crossword clue is the 8 letter word THIRTIES
I've seen this crossword clue in The Times
T
H
I
R
T
I
E
S
More clues leading to the result THIRTIES
Buddy explains!

The straight part
We know that "decade" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decade = thirties.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 THIRTIES mean?
THIRTIES noun- A decade starting with the year xx30, most usually the decade from 1930 to 1939.
- The decade of one's life from age 30 through age 39.
- (temperature, rates) The range between 30 and 39.
- A rack of thirty beers.