Beginning to doubt? (5,5)
I believe the answer is:
false start
'beginning to doubt?' is the definition.
(a beginning which should be doubted)
'beginning to doubt?' is the wordplay.
'beginning' becomes 'start' ('start' can be a synonym of 'beginning').
'to' says to put letters next to each other (I've seen this in other clues).
'doubt?' becomes 'false' (I can't justify this - if you can you should give a lot more credence to this answer).
'start' after 'false' is 'FALSE START'.
Can you help me to learn more?
(Other definitions for false start that I've seen before include "In which someone jumps the gun" , "may lead to total recall" , "In which runner is seen to have jumped the gun" , "Unsuccessful beginning" .)