Prospecting for oil is monotonous (6)
I believe the answer is:
boring
'monotonous' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for boring that I've seen before include "Tedious, of no interest" , "Making holes" , "like watching paint dry?" , "Tedious, tiresome" , "Tedious and tiring" .)