In fine fettle, yet initially unsure (4)
I believe the answer is:
iffy
'unsure' is the definition.
(I've seen this before)
'in fine fettle yet initially' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'in fine fettle yet' is 'IFFY'.
(Other definitions for iffy that I've seen before include "Doubtful (colloq.)" , "Doubtful, not definite" , "Questionable (inf)" , "Chancy, suspect" , "Not to be trusted" .)