Here the original manuscript appears to have read singled rather than single since both the 1830 edition and the printer’s manuscript (prior to Joseph Smith’s editing for the 1837 edition) had singled. The form singled could be an error for single, especially since in 3 Nephi 13:22, the other place in the text where this same phrase occurs, has single in both the 1830 edition and 𝓟 (“if therefore thine eye be single”), which is also the King James reading for Matthew 6:22. Nor does the Oxford English Dictionary list the verbal past participle singled as a variant for the adjective single. One possibility is that singled (which ends in a voiced alveolar stop) could be a mishearing resulting from single being immediately followed by to (which begins with a voiceless alveolar stop): “with an eye singled to his glory”.
Despite this evidence against singled, there are instances of the phrase “with an eye singled to his glory”, at least in recent English, especially in evangelical literature, as in these examples taken from :
For each of these cases, singled is used as the past participle for the verb single, with the meaning ‘to concentrate’; for this usage, see definition 8a under the verb single in the OED, which has the following citation that also uses singled (although in the simple past tense):
Thus the use of singled is possible in Mormon 8:15. The critical text will restore it, although it could be an error for single.
Summary: Restore in Mormon 8:15 the original form singled in the phraseology “with an eye singled to his glory”, the reading in 𝓟 and the 1830 edition (as well as in 𝓞, one would assume); singled could be an error for single, but it will nonetheless work with the meaning ‘concentrated’.