It took me a long time to figure this one out. Eight clues include codes that lead to the theme answer.
One way to enter a pool: FEETFIRST
One end of a church key: BOTTLEOPENER
Physical location?: MEDICALCENTER
Mobile communications device used in law enforcement: DATATERMINAL
Tchaikovsky’s 1812 Overture has one that includes cannon fire: BIGFINISH
Beyoncé hit with the repeated line "Say you'll never let me go": ENDOFTIME
Summing-up words: INCONCLUSION
A Guru whose opinions are trusted: THOUGHTLEADER
2019 Game of Thrones event: SERIESFINALE
Here we go. Are you sitting down?
The FIRST letter of FEET is F.
The OPENER of the word BOTTLE is B.
The CENTER of the word MEDICAL is I.
The TERMINAL letter of DATA is A.
The FINISH of the word BIG is G.
The END OF the word TIME is E.
The CONCLUSION of the word IN is N.
The LEADER of the word THOUGHT is T.
The FINALE of the word SERIES is S.
The government employees who do decoding are FBI AGENTS. Mark MacLachlan loves this kind of wordplay. His first published crossword (December 21, 2016) was titled "Aluminum Siding." It included several answers that begin or end with AL but the letters AL were not part of the grid. For example, one four-letter word was clued as "Director Hitchcock." The answer is ALFRED but only FRED will fit. The AL, if you write it at all, has to be off to the side. This man is clever!