Low-key lecturer dismissed by time theme can be discerned (7)

I believe the answer is:
subtext
'low-key lecturer' is the definition.
The definition suggests a singular noun which matches the answer.
'dismissed by time theme can be discerned' is the wordplay.
I cannot quite understand how this works, but
'dismissed' could be 'b' and 'b' is located in the answer.
'by' could be 'x' (2 by 4 is 2 x 4) and 'x' is located in the answer.
'time' could be 't' (abbreviation) and 't' is present in the answer.
The remaining letters 'suet' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for subtext that I've seen before include "Unwritten meaning" , "Level of deeper meaning" , "Implicit meaning" , "Something implied" , "Underlying theme" .)
