Not in the best of health? At first it’s fifty-fifty (3)
I believe the answer is:
ill
'not in the best of health?' is the definition.
(not in the best of health means unwell)
'at first it's fifty-fifty' is the wordplay.
'at' says to put letters next to each other.
'first' becomes 'i' (I in Roman numerals eg Elizabeth I).
'it's fifty-fifty' becomes 'LL' (L is 50 in Roman numerals).
'i' put next to 'll' is 'ILL'.
(Other definitions for ill that I've seen before include "In short, the state of Chicago" , "Dicky" , "Indisposed" , "Harmful" , "Unwell, sick" .)