Perhaps open a book in choppy sea as ill (10)

I believe the answer is:

assailable

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'perhaps open' is the definition.
'assailable' can be an answer for 'open' (synonyms). I'm not sure about the 'perhaps' bit.

'a book in choppy sea as ill' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'in' indicates putting letters inside.
'choppy' indicates an anagram (choppy implies disruption).
'a'+'b'='ab'
'sea'+'as'+'ill'='seaasill'
'seaasill' anagrammed gives 'assaille'.
'ab' put within 'assaille' is 'ASSAILABLE'.

Can you help me to learn more?

(Another definition for assailable that I've seen is " Open to attack".)

I've seen this clue in The Times.

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