Scientist almost entirely fixed up (5)
I believe the answer is:
tesla
'scientist' is the definition.
(I've seen this before)
'almost entirely fixed up' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'entirely' becomes 'all' (I've seen this before).
'fixed' becomes 'set' (synonyms).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'all' with its last letter removed is 'al'.
'al'+'set'='alset'
'alset' backwards is 'TESLA'.
(Other definitions for tesla that I've seen before include "SI unit; steal (anag.)" , "Nikola ---, electricity pioneer" , "Physicist" , "Unit of magnetic flux density" , "Physics unit" .)