The most common and most likely answer to this crossword clue is the 8 letter word COATTAIL
C
O
A
T
T
A
I
L
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The straight part
We know that "back flap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case back flap = coattail.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 COATTAIL mean?
COATTAIL noun- The flap at the back of a coat that hangs down, sometimes below the waist.
- (usually in the plural) Success of a figure, organization or movement that transfers to those who associate with it.
- To ride the coattails of.