Setter is past prime in stalemate (7)
I believe the answer is:
impasse
'stalemate' is the definition.
(synonyms)
'setter is past prime' is the wordplay.
'setter is' becomes 'I'm' (the setter is the person who set the crossword).
'past prime' becomes 'passe' (I can't justify this - if you can you should give a lot more credence to this answer).
'im'+'passe'='IMPASSE'
'in' is the link.
Can you help me to learn more?
(Other definitions for impasse that I've seen before include "Unpromising situation" , "Inescapable perdicament, dilemma" , "Stalemate situation" , "Lack of movement" , "Dead end" .)