Lubricates edges on gear's casing (7)
I believe the answer is:
greases
'lubricates' is the definition.
('grease' can be a synonym of 'lubricate')
'edges on gear's casing' is the wordplay.
'edges on' means to remove the middle letters.
'casing' becomes 'eases' (I can't justify this - if you can you should believe this answer much more).
'gear' with its middle removed is 'gr'.
'gr'+'eases'='GREASES'
Can you help me to learn more?
(Other definitions for greases that I've seen before include "One of those animal products" , "Lubricates" , "Bribes" .)