Slugs have to pull (3)
I believe the answer is:
lug
'pull' is the definition.
(I've seen this before)
'slugs have' is the wordplay.
'have' indicates the answer is hidden within the clue.
'LUG' is hidden in the letters of 'slugs'.
'to' acts as a link.
(Other definitions for lug that I've seen before include "Projecting piece; haul" , "Drag with effort; ear" , "Pull - ear" , "Drag heavily" , "Drag or carry with effort" .)