About fifty, one is plain absent-minded (9)
I believe the answer is:
oblivious
'absent-minded' is the definition.
(thesaurus)
'about fifty one is plain' is the wordplay.
'about' means one lot of letters goes inside another.
'fifty one' becomes 'li' (Roman numeral).
'is plain' becomes 'obvious' ('obvious' can be similar in meaning to 'plain').
'li' placed into 'obvious' is 'OBLIVIOUS'.
(Other definitions for oblivious that I've seen before include "Ignorant" , "like Lethe?" , "Heedless" , "Unconscious" , "Absent-mindedly unaware" .)