A slug on our ground is lethargic (10)
I believe the answer is:
languorous
'lethargic' is the definition.
(similar in meaning)
'a slug on our ground' is the wordplay.
'ground' indicates anagramming the letters ('grind' the letters into a new form).
'a'+'slug'+'on'+'our'='aslugonour'
'aslugonour' with letters rearranged gives 'LANGUOROUS'.
'is' is the link.
(Other definitions for languorous that I've seen before include "Full of dreamy inertia" , "Dreamy -- our UN goals (anag)" .)