Old plane not keeping constant speed (4)
I believe the answer is:
rate
'speed' is the definition.
(both can mean pace)
'old plane not keeping constant' is the wordplay.
'old plane' becomes 'crate' (informal term for an old vehicle).
'not keeping' indicates named letters should be taken away.
'constant' becomes 'c' (common symbol in maths for an unknown constant).
'crate' with 'c' taken out is 'RATE'.
(Other definitions for rate that I've seen before include "Rank; fee" , "Capital of the Philippines" , "Assign a value to something" , "See 11" , "Grade or evaluate" .)