Choose, after a month, to be ideally friendless! (6)
I believe the answer is:
decide
'choose' is the definition.
(I know that choose is a more specific form of the action decide)
'after a month to be ideally friendless' is the wordplay.
'after' means one lot of letters go next to another.
'a month' becomes 'Dec' (abbreviation for December).
'to be ideally friendless' becomes 'ide' (I can't explain this - if you can you should give a lot more credence to this answer).
'dec' put next to 'ide' is 'DECIDE'.
Can you help me to learn more?
(Other definitions for decide that I've seen before include "Resolve or give a judgment" , "Come to a resolution" , "Resolve; give judgement" , "Settle conclusively" , "Make up one's mind" .)