Girl’s description of operational ship impervious to attack (12)

I believe the answer is:

unassailable

'impervious to attack' is the definition.

'girl's description of operational ship' is the wordplay.
'girl's' becomes 'unas'.
'description of operational ship' becomes 'sailable'.
'unas'+'sailable'='UNASSAILABLE'

(Another definition for unassailable that I've seen is " Unable to be attacked".)

I've seen this clue in The Times.

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