Pursue stealthily (5)
'pursue stealthily' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stalk that I've seen before include "Follow sneakily" , "Stem; follow" , "Quietly pursue" , "Stem of plant" , "Follow prey stealthily" .)
(Other definitions for prowl that I've seen before include "Move stealthily (as tiger)" , "Move around restlessly" , "Steal around predatorily" , "Move about with unlawful intentions" , "Slink after prey" .)