Creature, two miles in, breaking limb (7)
I believe the answer is:
lemming
'creature' is the definition.
(lemming is a kind of animal)
'two miles in breaking limb' is the wordplay.
'two miles' can mean two replacements for 'miles'.
'miles' becomes 'm' (common abbreviation).
'miles' becomes 'm' (common abbreviation).
'breaking' indicates putting letters inside.
'limb' becomes 'leg' (leg is a kind of limb).
'm'+'m'+'in'='mmin'
'mmin' inserted into 'leg' is 'LEMMING'.
(Other definitions for lemming that I've seen before include "Self-destructive follower" , "Supposedly suicidal rodent" , "Rodent renowned for mass migration" , "Supposedly suicidal animal" , "Northern rodent" .)