Family member losing head in high spirits (7)
I believe the answer is:
elation
'high spirits' is the definition.
('elation' can be a synonym of 'high spirits')
'family member losing head' is the wordplay.
'family member' becomes 'relation' (I've seen this before).
'losing head' means to remove the first letter.
'relation' with its first letter taken away is 'ELATION'.
'in' is the link.
(Other definitions for elation that I've seen before include "State of pleasure about toe-nail" , "Feeling of delight or pride" , "'High spirits, exhilaration (7)'" , "Exaltation" , "Delight, thrill" .)