Five with chills and fever and it’s indefinite (5)
I believe the answer is:
vague
'it's indefinite' is the definition.
(synonyms)
'five with chills and fever' is the wordplay.
'five' becomes 'V' (Roman numeral).
'with' is a charade indicator (letters next to each other).
'chills and fever' becomes 'ague' (I've seen this before).
'v'+'ague'='VAGUE'
'and' is the link.
(Other definitions for vague that I've seen before include "Obscure, undefined" , "Indistinct or imprecise" , "Unclearly defined" , "Unclear" , "Not clearly expressed, woolly" .)