The most common and most likely answer to this crossword clue is the 5 letter word TENTH
T
E
N
T
H
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The straight part
We know that "a decade" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a decade = tenth.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 TENTH mean?
TENTH noun- The person or thing coming next after the ninth in a series; that which is in the tenth position.
- One of ten equal parts of a whole.
- The interval between any tone and the tone represented on the tenth degree of the staff above it, as between one of the scale and three of the octave above; the octave of the third.
- (in the plural) A temporary aid issuing out of personal property, and granted to the king by Parliament; formerly, the real tenth part of all the movables belonging to the subject.
- To divide by ten, into tenths.
- The ordinal numeral form of ten; next in order after that which is ninth.
- Being one of ten equal parts of a whole.