Think obsessively about diet, primarily, and about keeping fit (5,2)
I believe the answer is:
dwell on
'think' is the definition.
Although both the answer and definition are verbs in their base form, I don't understand how one could define the other.
'obsessively about diet primarily and about keeping fit' is the wordplay.
I cannot really see how this works, but
'about' could be 'on' (as in a book 'on' a certain topic) and 'on' is present in the answer.
'fit' could be 'well' (associated in meaning) and 'well' is located in the answer.
A single letter 'd' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for dwell on that I've seen before include "Think at length about" , "Speak at length about" , "Mull over" , "Ponder about" , "Linger over" .)