Think constantly about teacher keeping fit? (5,2)
I believe the answer is:
dwell on
'think' is the definition.
(I know that dwell on is a more specific form of the action the think)
'teacher keeping fit?' is the wordplay.
'teacher' becomes 'don' (don can mean an academic - especially at Oxbridge).
'keeping' means one lot of letters goes inside another (some letters kept inside).
'fit?' becomes 'well' ('well' is associated in meaning with 'fit').
'don' placed around 'well' is 'DWELL ON'.
'constantly about' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for dwell on that I've seen before include "Linger over" , "Mull over" , "Speak at length about" , "Think at length about" , "Ponder about" .)