A bit cruel to get plastered in oil? (9)
I believe the answer is:
lubricate
'oil?' is the definition.
(synonyms)
'a bit cruel to get plastered' is the wordplay.
'to get plastered' indicates an anagram (plastered can mean drunk).
'a'+'bit'+'cruel'='abitcruel'
'abitcruel' with letters rearranged gives 'LUBRICATE'.
'in' acts as a link.
(Other definitions for lubricate that I've seen before include "Make slippery" , "Oil (a mechanism)" , "supply with booze" , "Apply oil or grease to" , "Apply oil/grease" .)