Delight when family member loses head (7)
I believe the answer is:
elation
'delight' is the definition.
(I've seen this in another clue)
'family member loses head' is the wordplay.
'family member' becomes 'relation' (a relation or relative).
'loses head' means to remove the first letter.
'relation' with its first letter taken away is 'ELATION'.
'when' acts as a link.
(Other definitions for elation that I've seen before include "Feeling of delight, joy" , "State of being delighted or proud" , "'High spirits, exhilaration (7)'" , "Feeling of delight or pride" , "Transport" .)