Initially doctors diagnosed impetigo though it's only necessary to go on a supplement (8)
I believe the answer is:
addition
'supplement' is the definition.
(a supplement is added to something else)
'initially doctors diagnosed impetigo though it's only necessary to go on a' is the wordplay.
I cannot really see how this works, but
'doctors' could be 'dd' (two D's - D means doctor as in PhD) and 'dd' is present in the answer.
'on' is found in the answer.
'a' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for addition that I've seen before include "Calculation of the sum of two or more" , "Calculation of the sum of numbers" , "Arithmetic calculation" , "Reckoning" , "Arithmetical operation" .)