Impassive cathedral bigwig pocketing 500 per year (7)
I believe the answer is:
deadpan
'impassive' is the definition.
(I know that impassive can be written as deadpan)
'cathedral bigwig pocketing 500 per year' is the wordplay.
'cathedral bigwig' becomes 'dean' (dean is a type of clergyman).
'pocketing' is an insertion indicator (pocket can mean to steal or take possession of).
'500' becomes 'D' (Roman numeral for 500).
'per year' becomes 'p.a.' (synonyms).
'd'+'pa'='dpa'
'dean' placed around 'dpa' is 'DEADPAN'.
(Other definitions for deadpan that I've seen before include "Serious-looking" , "Blank" , "Keeping a straight face" , "showing no emotion" , "Poker-faced" .)