Unlikely story about returning ill, not to be relied on (8)
I believe the answer is:
fallible
'not to be relied on' is the definition.
I can't tell whether this definition defines the answer.
'unlikely story about returning ill' is the wordplay.
'unlikely story' becomes 'fable' (fable is a kind of story. I am not sure about the 'unlikely' bit.).
'about' means one lot of letters goes inside another.
'returning' says the letters should be written backwards.
'ill' backwards is 'lli'.
'fable' enclosing 'lli' is 'FALLIBLE'.
Can you help me to learn more?
(Other definitions for fallible that I've seen before include "apt to err" , "Liable to err" , "errant, possibly" , "Capable of being wrong" , "Capable of error, liable to make mistakes" .)