Great joy when kinsman loses his head? (7)
I believe the answer is:
elation
'great joy' is the definition.
(I've seen this before)
'kinsman loses his head?' is the wordplay.
'kinsman' becomes 'relation' (I've seen this before).
'loses his head?' means to remove the first letter.
'relation' with its first letter taken away is 'ELATION'.
'when' is the link.
(Other definitions for elation that I've seen before include "It's a great feeling!" , "State of pleasure about toe-nail" , "Exaltation" , "There's great joy" , "Transport" .)