Old reptile's power badly tore foot (11)
I believe the answer is:
pterodactyl
'old reptile's' is the definition.
(prehistoric reptile)
'power badly tore foot' is the wordplay.
'power' becomes 'p' (symbol in physics).
'badly' is an anagram indicator.
'foot' becomes 'dactyl' (dactyl is a kind of foot).
'tore' with letters rearranged gives 'tero'.
'p'+'tero'+'dactyl'='PTERODACTYL'
(Other definitions for pterodactyl that I've seen before include "Extinct flier" , "reptile from the past" , "Late Jurassic flyer" , "Winged reptile of old" , "old flying reptile" .)