Wish for hard old exercise (4)
I believe the answer is:
hope
'wish for' is the definition.
(I've seen this in another clue)
'hard old exercise' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'exercise' becomes 'pe' (Physical Education).
'h'+'o'+'pe'='HOPE'
(Other definitions for hope that I've seen before include "Anticipation" , "English-born US comedian, d. 2003" , "Bob --, US comedian" , "It springs eternal, it's said" , "The last thing in Pandora's Box" .)