Answer for alien's reserved but planned in advance (12)
I believe the answer is:
aforethought
'planned in advance' is the definition.
(I've seen this before)
'answer for alien's reserved but' is the wordplay.
I cannot really see how this works, but
'answer' could be 'a' (abbreviation eg in Q and A) and 'a' is present in the answer.
'but' could be 'though' (I've seen this before) and 'though' is found in the answer.
'alien' could be 'et' (extra terrestrial) and 'et' is present in the answer.
The remaining letters 'for' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for aforethought that I've seen before include "planned in advance" , "Conceived in advance" , "planned ahead" , "Premeditated" .)