Adventure novel unexpectedly about geese, nothing less (4,5)
I believe the answer is:
beau geste
'adventure novel' is the definition.
(novel by PC Wren)
'unexpectedly about geese nothing less' is the wordplay.
'unexpectedly' is an anagram indicator.
'nothing' becomes 'O' (looks like zero - 0).
'less' is a deletion indicator.
'about'+'geese'='aboutgeese'
'aboutgeese' with 'o' taken away is 'abutgeese'.
'abutgeese' is an anagram of 'BEAU GESTE'.
(Other definitions for beau geste that I've seen before include "Gracious act (Fr.); film" , "magnanimous act" , "Wren's work" , "Noble act; French foreign legion novel" .)