Fix because of flatness, say (4)
I believe the answer is:
tune
(Other definitions for tune that I've seen before include "temper" , "Air, melody" , "Adjust for better performance" , "Optimise the condition of" , "Adjust (car engine); air" .)
I believe the answer is:
tune
(Other definitions for tune that I've seen before include "temper" , "Air, melody" , "Adjust for better performance" , "Optimise the condition of" , "Adjust (car engine); air" .)