Crossword Genius

Underlying theme of U-boat book? (7)

Ross

I believe the answer is:

subtext

'underlying theme' is the definition.
(a secondary theme or message)

'u-boat book?' is the wordplay.
'u-boat' becomes 'sub' (German submarine).
'book?' becomes 'text' (text is a kind of book).
'sub'+'text'='SUBTEXT'

'of' acts as a link.

(Other definitions for subtext that I've seen before include "Unwritten meaning" , "Level of deeper meaning" , "Implicit meaning" , "Underlying theme" , "Something implied" .)

I've seen this clue in The Guardian.
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