Lacking in energy, gloomy and weighed down (5)
I believe the answer is:
laden
'weighed down' is the definition.
('laden' can be a synonym of 'weighed down')
'lacking in energy gloomy' is the wordplay.
'lacking in' is a deletion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'gloomy' becomes 'leaden'.
'leaden' with 'e' taken away is 'LADEN'.
'and' is the link.
(Other definitions for laden that I've seen before include "Encumbered" , "Heavily loaded" , "full cargo ship" , "Carrying a load" , "Weighed down with much cargo" .)