Covering letter enclosing lock of hair (7)
I believe the answer is:
ringlet
'lock of hair' is the definition.
(I've seen this before)
'covering letter enclosing' is the wordplay.
'enclosing' says the answer is hidden in the clue (the answer is enclosed by the clue).
'RINGLET' can be found hidden inside 'covering letter'.
(Other definitions for ringlet that I've seen before include "Cluster of hair" , "twist of sorts?" , "A curl - kind of tingler" , "Curling lock of hair" , "Hair feature" .)