Mixing lab acid with oil could be fiendish (10)
I believe the answer is:
diabolical
'be fiendish' is the definition.
(synonyms)
'mixing lab acid with oil' is the wordplay.
'mixing' is an anagram indicator.
'with' means one lot of letters go next to another.
'lab'+'acid'='labacid'
'labacid' put after 'oil' is 'oillabacid'.
'oillabacid' with letters rearranged gives 'DIABOLICAL'.
'could' is the link.
(Other definitions for diabolical that I've seen before include "Absolute" , "shocking" , "Devilish" , "Disgraceful" , "Fiendish" .)