Low dipstick gathering last drop of oil, visible when dark (7)
I believe the answer is:
moonlit
'visible when dark' is the definition.
'low dipstick gathering last drop of oil' is the wordplay.
'low' becomes 'moo' (both can mean to make the noise of a cow).
'dipstick' becomes 'nit' (both can mean a foolish person).
'gathering' means one lot of letters goes inside another.
'last drop of' says to take the final letters (I've seen 'last drops of' mean this).
The last letter of 'oil' is 'l'.
'nit' placed around 'l' is 'nlit'.
'moo'+'nlit'='MOONLIT'
(Other definitions for moonlit that I've seen before include "Illuminated by satellite" , "Gently illuminated" , "gently enlightened?" , "like Titania's unlucky meeting" , "Tom lion (anag)" .)