An opening word from Dr. Johnson (3)
I believe the answer is:
sir
'dr johnson' is the definition.
The definition suggests a singular noun which matches the answer.
'an opening word' is the wordplay.
'an' becomes 'i' (Roman numeral for one).
'opening' is an insertion indicator (some letters open a gap in others).
'word' becomes 'sr' (I can't explain this - if you can you should believe this answer much more).
'i' placed into 'sr' is 'SIR'.
'from' acts as a link.
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Title given to 10 Across" , "Honourable title" , "Term of address for a gentleman" , "Title of Kt or Bart" , "Way to address gentleman" .)