Energy shown by revellers at first in seedy bar (5)
I believe the answer is:
drive
'energy' is the definition.
(drive can mean energy or enthusiasm)
'revellers at first in seedy bar' is the wordplay.
'at first' says to take the initial letters.
'in' indicates putting letters inside.
'seedy bar' becomes 'dive' (as in a dive bar).
The first letter of 'revellers' is 'r'.
'r' placed into 'dive' is 'DRIVE'.
'shown by' is the link.
(Other definitions for drive that I've seen before include "Motivational stuff" , "Urge; approach to house" , "Urge along" , "Energy < tax" , "Steer; trip" .)