Deceptive serene exterior drinking spirits in RE (9)
I believe the answer is:
engineers
're' is the definition.
(Royal Engineers in the British Army)
'deceptive serene exterior drinking spirits' is the wordplay.
'deceptive' indicates an anagram.
'exterior' indicates putting letters inside (some letters go on the outside of others).
'drinking spirits' becomes 'gin' (gin is a spirit).
'serene' with letters rearranged gives 'eneers'.
'eneers' placed around 'gin' is 'ENGINEERS'.
'in' is the link.
(Other definitions for engineers that I've seen before include "They design roads, bridges etc." , "Contrives" , "Masterminds" , "Solvers of practical problems using scientific knowledge" , "They solve practical problems using scientific knowledge" .)