Brown sugar content in mugs exchanged for tablet (4)
I believe the answer is:
sear
'brown sugar' is the definition.
'sear' can be an answer for 'brown' (I've seen this in other clues). I am not sure about the 'sugar' bit.
'content in mugs exchanged for tablet' is the wordplay.
'content in' says to take the centre.
'mugs' becomes 'asses' (I've seen this in other clues).
'exchanged for tablet' becomes 'ear' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The middle letter of 'asses' is 's'.
's'+'ear'='SEAR'
Can you help me to learn more?
(Other definitions for sear that I've seen before include "Cauterise" , "Scorch or burn the surface of" , "Burn briefly" , "Dried-up, parched" , "Cook (quickly at great heat)" .)