Being always in extremes of solitude, is hard (6)
I believe the answer is:
severe
'is hard' is the definition.
(synonyms)
'being always in extremes of solitude' is the wordplay.
'being always' becomes 'ever' (I've seen this in another clue).
'in' is an insertion indicator.
'extremes of' means to remove the middle letters (only the letters at the extremes of the word).
'solitude' with its middle removed is 'se'.
'ever' going into 'se' is 'SEVERE'.
(Other definitions for severe that I've seen before include "Very bad" , "Ascetic - demanding - bitter" , "Extreme and difficult to endure" , "Grave" , "Harsh and punitive" .)