Craze always involves such keenness (4)
I believe the answer is:
zeal
'keenness' is the definition.
(zeal is a kind of keenness)
'craze always involves' is the wordplay.
'involves' says the answer is hidden in the clue.
'ZEAL' can be found hidden inside 'craze always'.
'such' acts as a link.
(Other definitions for zeal that I've seen before include "(Excessive) fervour" , "Extreme enthusiasm" , "Fervent devotion" , "Eager (if not fanatical) enthusiasm" , "Interest that's definitely not diminished" .)