Unable to write 49 + 51 three times a quarter? (10)
I believe the answer is:
illiterate
'unable to write' is the definition.
(illiterate means unable to read or write)
'49 51 three times a quarter?' is the wordplay.
'49' becomes 'IL' (Roman numerals).
'5a' becomes 'LI' (roman numeral).
'three' becomes 'ter' (see Chambers).
'times a quarter?' becomes 'ate' (I can't justify this - if you can you should give a lot more credence to this answer).
'il'+'li'+'ter'+'ate'='ILLITERATE'
(Other definitions for illiterate that I've seen before include "dunce" , "Ignorant" , "uneducated person" , "non-reader" , "Analphabetic" .)