Relax with partner after endless sex (6)
I believe the answer is:
sedate
'relax' is the definition.
'partner after endless sex' is the wordplay.
'partner' becomes 'date'.
'after' means one lot of letters go next to another.
'endless' means to remove the last letter.
'sex' with its last letter taken off is 'se'.
'date' after 'se' is 'SEDATE'.
'with' is the link.
(Other definitions for sedate that I've seen before include "Leisurely" , "Placid" , "Dignified - staid" , "Teased into being calm" , "Calm, not lively" .)