One flat, say, with important endorsement (3,9)
I believe the answer is:
key signature
'one flat say' is the definition.
(I know that key signature are often flat)
'with important endorsement' is the wordplay.
'with' means one lot of letters go next to another.
'important' becomes 'key' (similar in meaning).
'endorsement' becomes 'signature' (signing something can mean to endorse it).
'key' next to 'signature' is 'KEY SIGNATURE'.
(Other definitions for key signature that I've seen before include "musical reference" , "(In written music) an instruction involving sharps and flats" , "Musical stave's collection of sharps or flats" , "For example, at the beginning of a stave line two sharps for D major" , "This fixes pitches" .)