Internet pest missing start of leisurely walk (5)
I believe the answer is:
troll
'internet pest' is the definition.
'missing start of leisurely walk' is the wordplay.
'missing start of' means to remove the first letter.
'leisurely walk' becomes 'stroll' (I've seen this before).
'stroll' with its initial letter taken away is 'TROLL'.
(Other definitions for troll that I've seen before include "supposedly lives in a cave" , "Possibly a monster" , "Fairytale monster" , "Fish - gnome - sing" , "Internet provocateur" .)