Cowboy is awkward having expended energy for nothing (6)
I believe the answer is:
gaucho
'cowboy' is the definition.
(term for a South American cowboy)
'awkward having expended energy for nothing' is the wordplay.
'awkward' becomes 'gauche' (I've seen this before).
'having expended' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'for' says to put letters next to each other.
'nothing' becomes 'O' (looks like zero - 0).
'gauche' with 'e' taken away is 'gauch'.
'gauch'+'o'='GAUCHO'
'is' is the link.
(Other definitions for gaucho that I've seen before include "Horseman of the Pampas" , "Cowboy of Argentine's plains" , "Plain-living person" , "Cowboy of the South American pampas" , "plainsman" .)