The most common and most likely answer to this crossword clue is the 5 letter word NATES
N
A
T
E
S
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The straight part
We know that "Cushion at the rear, cheeky seat?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cushion at the rear, cheeky seat? = nates.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 NATES mean?
NATES noun- The two anterior of the four lobes on the dorsal side of the midbrain of most mammals; the anterior optic lobes.
- The buttocks.
- The umbones of a bivalve shell.