Companion is dull with little energy (4)
I believe the answer is:
mate
'companion' is the definition.
(I've seen this before)
'dull with little energy' is the wordplay.
'dull' becomes 'mat' ('mat' can be similar in meaning to 'dull'**).
'with' says to put letters next to each other.
'little' indicates taking the first letters.
The initial letter of 'energy' is 'e'.
'mat'+'e'='MATE'
'is' acts as a link.
(Other definitions for mate that I've seen before include "China" , "Marry - friend" , "Chess finale" , "Chess ending; pal" , "Comrade - spouse - marry" .)