Woman trapped by lustful Roman Catholic priest (8)
I believe the answer is:
cardinal
'roman catholic priest' is the definition.
'woman trapped by lustful' is the wordplay.
'woman' becomes 'di' (short for Diana).
'trapped by' is an insertion indicator.
'lustful' becomes 'carnal'.
'di' going into 'carnal' is 'CARDINAL'.
(Other definitions for cardinal that I've seen before include "Key" , "Primary - colour" , "cleric" , "Prince of the RC church" , "Leading RC dignitary" .)