Think about Mafia boss keeping fit (5,2)
I believe the answer is:
dwell on
'think about' is the definition.
(to dwell on something is to think about it)
'mafia boss keeping fit' is the wordplay.
'mafia boss' becomes 'don' (term for a mafia leader).
'keeping' is an insertion indicator.
'fit' becomes 'well' (associated in meaning).
'don' placed around 'well' is 'DWELL ON'.
(Other definitions for dwell on that I've seen before include "Ponder about" , "Speak at length about" , "Linger over" , "Mull over" , "Think at length about" .)