Five having fever hard to pin down (5)
I believe the answer is:
vague
'hard to pin down' is the definition.
'five having fever' is the wordplay.
'five' becomes 'v' (roman numeral).
'having' says to put letters next to each other.
'fever' becomes 'ague' (I've seen this in other clues).
'v'+'ague'='VAGUE'
(Other definitions for vague that I've seen before include "Ill-defined" , "Unclearly defined" , "Indeterminate" , "Unclear" , "Hazy" .)