The most common and most likely answer to this crossword clue is the 9 letter word MEMBRANES
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The straight part
We know that "films" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case films = membranes.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 MEMBRANES mean?
MEMBRANES noun- A flexible enclosing or separating tissue forming a plane or film and separating two environments (usually in a plant or animal).
- A mechanical, thin, flat flexible part that can deform or vibrate when excited by an external force.
- A flexible or semi-flexible covering or waterproofing whose primary function is to exclude water.