Poor 'rithmetic perhaps later discovered (5-4)
I believe the answer is:
third-rate
'poor' is the definition.
(I've seen this before)
''rithmetic perhaps later discovered' is the wordplay.
''rithmetic' becomes 'thirdr' (I've seen this in another clue).
'perhaps' becomes 'at' (I can't explain this - if you can you should believe this answer much more).
'later' becomes 'someday' (some day in the future).
'discovered' means to look at the middle letters (dis-covered - outer letters removed).
The middle letter of 'someday' is 'e'.
'thirdr'+'at'+'e'='THIRD-RATE'
Can you help me to learn more?
(Other definitions for third-rate that I've seen before include "Of very poor quality" , "Very inderior in quality - do a count" , "Very inferior in quality in numeric description" , "Very inferior in numerical way" , "Very inferior in quality, using ordinal number" .)