All but a quarter premature (6)
I believe the answer is:
nearly
'all but' is the definition.
(thesaurus)
'a quarter premature' is the wordplay.
'a quarter' becomes 'n'.
'premature' becomes 'early' ('early' can be similar in meaning to 'premature').
'n'+'early'='NEARLY'
(Other definitions for nearly that I've seen before include "Give out a bright light" , "pretty much" , "parsimoniously" , "More or less" , "Close" .)