Very keen psalmist loses his head (4)
I believe the answer is:
avid
'very keen' is the definition.
(I've seen this before)
'psalmist loses his head' is the wordplay.
'psalmist' becomes 'david' (I've seen this before).
'loses his head' means to remove the first letter.
'david' with its initial letter taken off is 'AVID'.
(Other definitions for avid that I've seen before include "Greedy - enthusiastic" , "Eager like diva" , "Keenly interested" , "'Eager, zealous (4)'" , "deeply enthusiastic" .)