Note the old ingenuity on head of one who saw (3-7)
I believe the answer is:
eye-witness
'one who saw' is the definition.
The definition suggests a singular noun which matches the answer.
'note the old ingenuity on head' is the wordplay.
'note' becomes 'e' (musical note).
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'ingenuity' becomes 'wit' (wit can mean cleverness or ingenuity).
'on head' becomes 'ness' (ness is a headland. I am not sure about the 'on' bit.).
'e'+'ye'+'wit'+'ness'='EYE-WITNESS'
'of' is the link.
Can you help me to learn more?
(Other definitions for eye-witness that I've seen before include "Spectator" , "one observes" .)