Musk defending lab's closure over fine (5)
I believe the answer is:
noble
'fine' is the definition.
(I've seen this before)
'musk defending lab's closure over' is the wordplay.
'musk' becomes 'Elon' (Elon Musk).
'defending' means one lot of letters goes inside another (inserted letters are defended).
'closure' indicates one should take the final letters (the letter which closes the word).
'over' shows that the letters should be reversed in order.
The final letter of 'lab' is 'b'.
'elon' placed around 'b' is 'elbon'.
'elbon' in reverse letter order is 'NOBLE'.
(Other definitions for noble that I've seen before include "Of high rank or birth" , "Having high moral qualities" , "Lord" , "Eminent" , "Stately" .)