Fervent giant-killer loses his head (4)
I believe the answer is:
avid
'fervent' is the definition.
(I've seen this in another clue)
'giant-killer loses his head' is the wordplay.
'giant killer' becomes 'david' (David killed Goliath in the Bible).
'loses his head' means to remove the first letter.
'david' with its first letter taken off is 'AVID'.
(Other definitions for avid that I've seen before include "Keen or greedy" , "Keen, enthusiastic" , "'Eager, enthusiastic (4)'" , "Greedy like diva" , "Greedily eager" .)