Impassive cathedral dignitary detaining Dutch father (7)
I believe the answer is:
deadpan
'impassive' is the definition.
(I know that impassive can be written as deadpan)
'cathedral dignitary detaining dutch father' is the wordplay.
'cathedral dignitary' becomes 'dean' (I've seen this before).
'detaining' means one lot of letters goes inside another.
'dutch' becomes 'd' (common abbreviation).
'father' becomes 'pa' (pa can mean dad or father).
'd'+'pa'='dpa'
'dean' placed around 'dpa' is 'DEADPAN'.
(Other definitions for deadpan that I've seen before include "with emotionless face" , "Showing no expression" , "With expressionless face" , "Keeping a straight face" , "Poker-faced" .)