I am not entirely convinced by the solution proposed by the auctioneer is correct, also see Mary Beard’s article from her TLS blog.

Edward Lear, The Rebus Letter: I would be a great arse in writing should I not attempt to afford to catch the muse.
Dated ’23 August’. Pen and ink. 4 ¼ x 5 inches.

A rebus is a text that is presented as a combination of images and individual letters. The form may have originated in Egypt as early as 3400 BC, but was certainly popular in England during the eighteenth and nineteenth centuries, both in correspondence and as an entertainment. The present example by Edward Lear may be either the beginning of a letter or an element in a game. It spells out the following message: ‘I would be a great arse in/writing should I not attempt/to afford to catch the muse’. The use of the word ‘arse’ may seem out of character for such a writer as Lear. However, it does appear in his diaries, including the entry for 1 September 1867, when he relays the words of a German tutor spoken during a game of Charades: ‘it is Arse: for it says – let it be concealed – certainly I should always conceal my arse: nobody should show his arse’.