Rare single: not running in between (8)
I believe the answer is:
unwonted
'rare' is the definition.
(unwonted can mean rare or unusual)
'single not running in between' is the wordplay.
'single' becomes 'unwed' (synonyms).
'running' indicates an anagram (the letters run around to different places).
'in between' indicates putting letters inside.
'not' anagrammed gives 'ont'.
'unwed' placed around 'ont' is 'UNWONTED'.
(Other definitions for unwonted that I've seen before include "Out of the ordinary, uncommon" , "Unaccustomed" , "not typical" , "Not habitually" , "Atypical, not usual" .)