Hard to believe what a long way it's come! (3-7)
I believe the answer is:
far-fetched
'hard to believe' is the definition.
I can't tell whether this defines the answer.
'a long way it's come' is the wordplay.
'a long way' becomes 'far' (I've seen this in other clues).
'it's come' becomes 'fetched' (I can't explain this - if you can you should believe this answer much more).
'far'+'fetched'='FAR-FETCHED'
'what' is the link.
Can you help me to learn more?
(Other definitions for far-fetched that I've seen before include "Highly implausible" , "Incredible" , "(Tale) hard to believe" , "Forced - unconvincing" , "Unlikely" .)