Agreed to be the product of a single thinker (2,3,4)
I believe the answer is:
of one mind
'agreed' is the definition.
(I've seen this before)
'of a single thinker' is the wordplay.
'a single' becomes 'one' (a single thing - only one).
'thinker' becomes 'mind' (synonyms).
'of'+'one'+'mind'='OF ONE MIND'
'to be the product' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for of one mind that I've seen before include "Unanimous" , "In accord" , "In agreement" , "Agreed" , "Sharing an opinion" .)