Sister found in company oil container — that's a puzzle! (9)
I believe the answer is:
conundrum
'that's a puzzle' is the definition.
(a conundrum is a puzzle or mystery)
'sister found in company oil container' is the wordplay.
'sister' becomes 'nun' (sister is a term of address for a nun).
'found in' is an insertion indicator.
'company' becomes 'co' (abbreviation).
'oil container' becomes 'drum' (I've seen this in another clue).
'co'+'drum'='codrum'
'nun' put inside 'codrum' is 'CONUNDRUM'.
(Other definitions for conundrum that I've seen before include "anagram" , "Puzzling problem" , "Poser" , "That's puzzling" , "It's a riddle or a hard question" .)