Isn't able to be in the reserves - has second thoughts (7)
I believe the answer is:
recants
'reserves has second thoughts' is the definition.
I can't judge whether this definition defines the answer.
'isn't able to be in' is the wordplay.
'isn't able' becomes 'can't' (I've seen this in another clue).
'to' becomes 'res' (I can't explain this - if you can you should give a lot more credence to this answer).
'be in' indicates putting letters inside.
'cant' placed into 'res' is 'RECANTS'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for recants that I've seen before include "Disclaims, takes back" , "takes back what was said" , "Withdraws declaration" , "Retracts" .)