Dark energy familiar, not entirely (6)
I believe the answer is:
gothic
'dark' is the definition.
'energy familiar not entirely' is the wordplay.
'energy' becomes 'go' (I've seen this in other clues).
'familiar' becomes 'thick' (both can mean having a close relationship).
'not entirely' means to remove the last letter.
'thick' with its last letter taken off is 'thic'.
'go'+'thic'='GOTHIC'
(Other definitions for gothic that I've seen before include "Old-fashioned type" , "Pointed-arch style" , "Architectural style in Europe 12th-16th centuries" , "Style of architecture and gloomy books" , "Style of mediaeval architecture" .)