Six clues have thematic answers. The remaining across clues have an extra letter in the wordplay
10a Cured fish reek, "Do heed Rory," I'd nag from time to time (10, 2 words)
O?DLE??M?S
1d Millions Vagans to pledge as wager in error of judgment (Vagans is the setter) (7)
C?AR???
17d Promissory notes subject to rejection: removing note that's forged (8) ?E?E??O?
For 10a the answer is given in the clue by alternate letters from the R in Reek... (two words 3,7).
17d the answer is 8 letters - it ends with the usual 4 letters for 'promissory notes', ['subject to'] a 5 letter word for 'rejection' (in the unusual nounal sense, more usually it would be 'to reject') minus its final 'n'/note - def is 'forged' as in fake.