Unpopular day, interminable (10)
I believe the answer is:
friendless
'unpopular' is the definition.
(I've seen this before)
'day interminable' is the wordplay.
'day' becomes 'fri'.
'interminable' becomes 'endless' ('endless' can be a synonym of 'interminable').
'fri'+'endless'='FRIENDLESS'
(Other definitions for friendless that I've seen before include "Lacking pals" , "Socially alone" , "lonely" , "solitary" , "Unpopular" .)