Lubricate old rifle regularly (3)
I believe the answer is:
oil
'lubricate' is the definition.
(synonyms)
'old rifle regularly' is the wordplay.
'old' becomes 'o' (abbreviation).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'rifle' are 'il'.
'o'+'il'='OIL'
(Other definitions for oil that I've seen before include "Liquid lubricant" , "Type of fuel" , "Viscous substance as lubricant" , "painting" , "Lubricating liquid" .)