Sloth, one in a tree, almost moving (7)
I believe the answer is:
inertia
'sloth' is the definition.
(inertia can mean laziness or sloth)
'one in a tree almost moving' is the wordplay.
'one' becomes 'i' (Roman numeral).
'almost' means to remove the last letter (most of the word but not all of it).
'moving' is an anagram indicator (letters are moved around).
'in'+'a'+'tree'='inatree'
'inatree' with its last letter removed is 'inatre'.
'inatre' anagrammed gives 'nertia'.
'i'+'nertia'='INERTIA'
(Other definitions for inertia that I've seen before include "Disposition to do nothing" , "Lack of movement" , "Admonish" , "Apathy" , "Slowness to act, lassitude" .)