It is based on an earlier design (13)
I believe the answer is:
premeditation
'earlier design' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'it is based on' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
'on' could be 'at' (I've seen this in other clues) and 'at' is found within the answer.
'is' could be 'ii' (more than one I) and 'ii' is found in the remaining letters.
This may be the basis of the clue (or it may be nonsense).
'an' acts as a link.
Can you help me to learn more?
(Another definition for premeditation that I've seen is " Forethought".)