Lock of hair found inside covering letter (7)
I believe the answer is:
ringlet
'lock of hair' is the definition.
(I've seen this before)
'found inside covering letter' is the wordplay.
'found' indicates the answer is hidden within the clue.
'RINGLET' is hidden in the letters of 'inside covering letter'.
(Other definitions for ringlet that I've seen before include "Cluster of hair" , "Curl, of an Irish dancer say" , "A curl - kind of tingler" , "twist of sorts?" , "Curled lock of hair" .)