Nut, losing head, with shift of active enthusiasm (4)
I believe the answer is:
zeal
'enthusiasm' is the definition.
(I know that zeal is a type of avidity)
'nut losing head with shift of active' is the wordplay.
I cannot really understand how this works, but
'active' could be 'a' (grammatical abbreviation) and 'a' is found in the answer.
The remaining letters 'zel' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for zeal that I've seen before include "Intense enthusiasm" , "Eager (if not fanatical) enthusiasm" , "Zing" , "Zest, ardour" , "Verve" .)