Clergyman brings in a pile (7)
I believe the answer is:
reactor
'pile' is the definition.
(I've seen this before)
'clergyman brings in a' is the wordplay.
'clergyman' becomes 'rector' (rector is a kind of clergyman).
'brings in' is an insertion indicator.
'rector' enclosing 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "device for generating energy" , "one hosting moderator?" , "nuclear facility" , "Nuclear apparatus for producing energy" , "chemical substance!" .)