Live healthily after start of diet (5)
I believe the answer is:
dwell
'live' is the definition.
(to live/dwell somewhere)
'healthily after start of diet' is the wordplay.
'healthily' becomes 'well' (as in eating well).
'after' says to put letters next to each other.
'start of' says to take the initial letters.
The first letter of 'diet' is 'd'.
'well' after 'd' is 'DWELL'.
(Other definitions for dwell that I've seen before include "Focus attention (on)" , "Hang out" , "Harp" , "Settle" , "Reside - linger" .)