Hard reading about arithmetic? (6)
I believe the answer is:
sturdy
'hard' is the definition.
(thesaurus)
'reading about arithmetic?' is the wordplay.
'reading' becomes 'r' (one of the traditional three R's of education).
'about' means one lot of letters goes inside another.
'arithmetic?' becomes 'study' (arithmetic is a kind of study).
'r' put inside 'study' is 'STURDY'.
(Other definitions for sturdy that I've seen before include "Firmly constructed" , "Strong and reliable" , "Well-built - strapping" , "Strong, solid" , "Strong and sturdily built" .)