Fahrenheit alternative (7)
I believe the answer is:
celsius
'fahrenheit alternative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for celsius that I've seen before include "Temperature scale named after Swedish astronomer" , "Scale of temperature devised by Swedish astronomer" , "Swedish astronomer, deviser of the centigrade thermometer" , "Scale of temperature devised by Swiss astronomer" , "alternative to F spelt out?" .)