Wearing down gearbox occasionally -- is reversing working (7)
I believe the answer is:
erosion
'wearing down' is the definition.
(I've seen this before)
'gearbox occasionally is reversing working' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'reversing' is a reversal indicator.
'working' becomes 'on' (on can mean working or activated).
The alternating letters of 'gearbox' are 'ero'.
'is' reversed gives 'si'.
'ero'+'si'+'on'='EROSION'
(Other definitions for erosion that I've seen before include "The wearing away as by wind or water" , "Disintegration" , "dying away" , "Abrasion" , "Process of wear" .)