Sloth I train somehow round back of menagerie (7)
I believe the answer is:
inertia
'sloth' is the definition.
(I've seen this before)
'i train somehow round back of menagerie' is the wordplay.
'somehow' indicates anagramming the letters.
'round' indicates putting letters inside.
'back of' says to take the final letters.
The final letter of 'menagerie' is 'e'.
'train' is an anagram of 'nrtia'.
'nrtia' enclosing 'e' is 'nertia'.
'i'+'nertia'='INERTIA'
(Other definitions for inertia that I've seen before include "Change-resisting tendency" , "Resistance to change" , "Resistance to motion" , "Lack of inclination to move" , "I earn it (anag.)" .)