Like die-hard fans (4)
I believe the answer is:
avid
'like die-hard fans' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for avid that I've seen before include "Showing active interest and enthusiasm" , "'Eager, zealous (4)'" , "Keenly enthusiastic" , "Greatly desirous" , "Keen and eager" .)