Problem with sister getting in fake spirit (9)
I believe the answer is:
conundrum
'problem' is the definition.
(I know that conundrum is a type of problem)
'sister getting in fake spirit' is the wordplay.
'sister' becomes 'nun' (sister is a term of address for a nun).
'getting in' means one lot of letters goes inside another.
'fake' becomes 'cod' (cod can mean false or inauthentic).
'spirit' becomes 'rum' (alcoholic spirit).
'nun' inserted inside 'cod' is 'conund'.
'conund'+'rum'='CONUNDRUM'
'with' acts as a link.
(Other definitions for conundrum that I've seen before include "That's puzzling" , "Puzzle, riddle" , "Difficult question" , "Puzzling question" , "Poser" .)