Fifth old book order library finally overlooked? Something small being charged (8)

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

definition: deuteron is the nucleus of heavy hydrogen, and has a positive charge DEUTERON-[OM Y] is the fifth book of the Old Testament, minus OM Y as these are “overlooked” OM (Order of Merit) and Y as “librar-Y finally”