Sign there's some shale oil (3)
I believe the answer is:
leo
'sign' is the definition.
(star sign)
'some shale oil' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'LEO' is hidden in the letters of 'shale oil'.
'there's' acts as a link.
(Other definitions for leo that I've seen before include "house" , "Zodiacal lion" , "one of several popes" , "sign of summer" , "Lion constellation" .)