Stalk and hard nut to crack (4)
I believe the answer is:
hunt
'stalk' is the definition.
(I've seen this in another clue)
'hard nut to crack' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to crack' is an anagram indicator (I've seen 'cracked' mean this).
'nut' with letters rearranged gives 'unt'.
'h'+'unt'='HUNT'
'and' is the link.
(Other definitions for hunt that I've seen before include "Pursue game" , "Racing driver" , "Quest" , "Chase and kill" , "Forage" .)