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

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

SUBT [l] E (low-key) without the L (abbreviation for lecturer), then X (mathematical symbol for multiplication = by, as in “3 by 2” = 3 x 2) + T (abbreviation for time). The underlying motivation behind something said or written, which isn’t stated explicitly but can be understood.