Fashionable people in command producing tension (5)
I believe the answer is:
tonic
'producing tension' is the definition.
(tonic can mean producing tension in muscles)
'fashionable people in command' is the wordplay.
'fashionable people' becomes 'ton' ('the ton' could once mean fashionable society).
'in command' becomes 'ic' (military abbreviation).
'ton'+'ic'='TONIC'
(Other definitions for tonic that I've seen before include "that may be mixed with gin" , "Stimulus" , "Invigorating drink" , "Accompaniment to gin?" , "Fillip" .)