Answering letter enclosing lock of hair (7)
I believe the answer is:
ringlet
'lock of hair' is the definition.
(I've seen this before)
'answering letter enclosing' is the wordplay.
'enclosing' indicates the answer is hidden within the clue (the answer is enclosed by the clue).
'RINGLET' is hidden within 'answering letter'.
(Other definitions for ringlet that I've seen before include "Curled lock of hair" , "Curl, of an Irish dancer say" , "Little curl" , "Cluster of hair" , "Curl of hair; butterfly" .)