Number get a fever that's ill-defined (5)
I believe the answer is:
vague
'ill-defined' is the definition.
(synonyms)
'number get a fever' is the wordplay.
'number' becomes 'v' (V is an example).
'get' is a charade indicator (letters next to each other).
'a fever' becomes 'ague' (illness causing fever).
'v'+'ague'='VAGUE'
'that's' acts as a link.
(Other definitions for vague that I've seen before include "Unclear" , "Unclearly defined" , "Nebulous" , "Indeterminate" , "Not clearly expressed or perceived" .)