Salt content of tobacco (3)
I believe the answer is:
tar
'salt content of tobacco' is the definition.
'tar' can be an answer for 'salt' (I've seen this before). I'm unsure of the 'content of tobacco' bit.
'salt content of tobacco' is the wordplay.
'salt' becomes 'niter' (niter is a kind of salt).
'content' indicates the central letters.
'of tobacco' becomes 'ar' (I can't justify this - if you can you should give a lot more credence to this answer).
The centre of 'niter' is 't'.
't'+'ar'='TAR'
Can you help me to learn more?
(Other definitions for tar that I've seen before include "Sticky substance used in road construction" , "Jack ..., British sailor" , "Dark viscid substance" , "Dark flammable liquid" , "Dark viscous flammable substance" .)