Unit worry over resistance award (8)

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

ANGST (worry) before (over, in a down clue) R (abbreviation for electrical resistance) + OM (abbreviation for Order of Merit = award). Unit used to express the wavelengths of electromagnetic radiation (one ten thousand millionth of a metre), named after the Swedish physicist Anders Jonas Ångström.