Trained person needs to write the template (9)
I believe the answer is:
prototype
'template' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'trained person needs to write' is the wordplay.
'trained person' becomes 'pro' (pro is a kind of person. I am not sure about the 'trained' bit.).
'needs' is a charade indicator (letters next to each other).
'write' becomes 'type' (typing is a kind of writing).
'pro'+'to'+'type'='PROTOTYPE'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for prototype that I've seen before include "First version (of a device) built for development" , "Example for testing" , "Preliminary build; mock-up" , "First full-scale working model" , "Preliminary version from which other forms may be developed" .)