Chemical entity close to another one and one or other close to another (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

I (one in Roman numerals) + SOME (one or other = an unspecified person or thing) + last letter (close) of [anothe] R . Isomers = substances that have the same chemical formula but different arrangements of atoms (and therefore may have different physical or chemical properties).