Line on taxi mostly grease (4)
I believe the answer is:
lube
'grease' is the definition.
'line on taxi mostly' is the wordplay.
'line' becomes 'l'.
'on' is a charade indicator (letters next to each other).
'taxi' becomes 'uber'.
'mostly' means to remove the last letter.
'uber' with its last letter taken away is 'ube'.
'l'+'ube'='LUBE'
(Other definitions for lube that I've seen before include "oil" , "Grease (abbr)" .)