Clergyman pockets a pile (7)
I believe the answer is:
reactor
'pile' is the definition.
(term for a nuclear reactor)
'clergyman pockets a' is the wordplay.
'clergyman' becomes 'rector' (rector is a kind of clergyman).
'pockets' indicates putting letters inside.
'rector' going around 'a' is 'REACTOR'.
(Other definitions for reactor that I've seen before include "nuclear facility" , "device for generating energy" , "Nuclear power station core" , "Nuclear apparatus for producing energy" , "Nuclear installation" .)