i think the 'a' is equal to 'some' [indefinite article] with nag both reversed [ie about] which gives gana [the leading 'n' being omitted. spun is as you have it. cheers
Ixion has the right wordplay, in my view. 36 Ac being the only word which has 2 of the missing letters. Very kind of Columba to have them all together and actually spelling out the required word in the correct order. Once you'd sussed the theme from the first 5 letters and the title, the first 2 letters of 36ac had to be N&G, leaving you with ANA from the wordplay as explained by Ixion.
Looking at it again I'm not sure that bellabee is right. The preamble states seven clues must be used if we take two letters from one clue surely only six are used?