Relative, sprightly, heading off (5)
I believe the answer is:
aunty
'relative' is the definition.
(aunty is a kind of relative)
'sprightly heading off' is the wordplay.
'sprightly' becomes 'jaunty' (I've seen this in another clue).
'heading off' means to remove the first letter.
'jaunty' with its first letter taken away is 'AUNTY'.
(Other definitions for aunty that I've seen before include "A parent's sister" , "Female relative" , "Joined by this relation" .)