A little sun: fair (6)
I believe the answer is:
slight
'a little' is the definition.
(thesaurus)
'sun fair' is the wordplay.
'sun' becomes 'S' (abbreviation).
'fair' becomes 'light' (as in fair hair).
's'+'light'='SLIGHT'
(Other definitions for slight that I've seen before include "Insubstantial; discourtesy" , "Deliberate rebuff" , "of no great significance" , "A disrespectful reference - not great" , "Slim or tenuous" .)