Tendon he damaged covering miles is healing (2,3,4)
I believe the answer is:
on the mend
'is healing' is the definition.
(I've seen this in another clue)
'tendon he damaged covering miles' is the wordplay.
'damaged' is an anagram indicator.
'covering' indicates putting letters inside.
'miles' becomes 'm' (abbreviation).
'tendon'+'he'='tendonhe'
'tendonhe' is an anagram of 'ontheend'.
'ontheend' enclosing 'm' is 'ON THE MEND'.
(Other definitions for on the mend that I've seen before include "returning to health" , "Regaining health" , "Informally getting better, improving after illness or accident" , "undergoing recovery?" , "Making recovery" .)