Difficult to conceal source of oil supply (5)
I believe the answer is:
hoard
'oil supply' is the definition.
'hoard' can be an answer for 'supply' (I've seen this before). I'm unsure of the 'oil' bit.
'difficult to conceal source of' is the wordplay.
'difficult' becomes 'hard' ('hard' can be a synonym of 'difficult').
'to conceal' means one lot of letters goes inside another.
'source' suggests taking the first letters (I've seen 'source of' mean this).
The first letter of 'of' is 'o'.
'hard' placed around 'o' is 'HOARD'.
Can you help me to learn more?
(Other definitions for hoard that I've seen before include "Collection" , "Cache" , "Store of treasure" , "One's savings" , "Squirrel away" .)