Possibly infallible start, in the end (5)
I believe the answer is:
final
'the end' is the definition.
(I've seen this before)
'possibly infallible start in' is the wordplay.
I cannot quite see how this works, but
'in' is within the answer.
The remaining letters 'fal' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for final that I've seen before include "big match" , "Not to be altered" , "Like one's journey to cemetery" , "Last match in a competition" , "Last in a series of games or contests" .)