Cover coolants regularly going missing (4)
I believe the answer is:
coat
'cover' is the definition.
(coat is a kind of cover)
'coolants regularly going missing' is the wordplay.
'regularly going missing' indicates alternate letters.
The alternating letters of 'coolants' are 'COAT'.
(Other definitions for coat that I've seen before include "Apply layer" , "Layer of paint" , "Outer garment" , "Overgarment" , "Warm outerwear" .)