A right pair mostly exercise hard (7)
I believe the answer is:
arduous
'hard' is the definition.
(I know that hard can be written as arduous)
'a right pair mostly exercise' is the wordplay.
'right' becomes 'r' (common abbreviation).
'pair' becomes 'duo' (two people).
'mostly' means to remove the last letter (most but not all of the word).
'exercise' becomes 'ous' (I am not sure about this - if you are sure you should believe this answer much more).
'duo' with its last letter taken away is 'du'.
'a'+'r'+'du'+'ous'='ARDUOUS'
Can you help me to learn more?
(Other definitions for arduous that I've seen before include "Burdensome" , "Toilsome" , "Tough" , "Steep - hard" , "Difficult, tiring" .)