Frenzy? Not so many, when half-hearted? (5)
I believe the answer is:
fever
'frenzy?' is the definition.
(I've seen this before)
'when half-hearted?' is the wordplay.
'when' becomes 'if'.
'half' indicates taking half.
'hearted?' becomes 'ever' (I can't explain this - if you can you should believe this answer much more).
'if' with half the letters taken is 'f'.
'f'+'ever'='FEVER'
'not so many' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
(Other definitions for fever that I've seen before include "Intense nervous excitement or anticipation?" , "Flu symptom: frenzy" , "Ague, abnormally high body temperature" , "Nervous excitement, high temperature" , "A rise in the temperature of the body" .)