Septimus, an excellent back, adds a certain tone (5)
I believe the answer is:
sepia
'tone' is the definition.
'septimus an excellent back adds a certain' is the wordplay.
I cannot really understand how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is found within the answer.
'a' is present in the answer.
The remaining letters 'sep' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sepia that I've seen before include "Colour from cuttlefish ink" , "Denizen of deep" , "source of ink" , "Pigment obtained from cuttlefish ink" , "Old drawing" .)