Underlying theme of U-boat book? (7)
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" .)