Think about a change of diet, chum (8)
I believe the answer is:
'think' is the definition.
(I know this)
'about a change of diet chum' is the wordplay.
'about' is an insertion indicator.
'a change of' indicates anagramming the letters.
'chum' becomes 'mate' (I've seen this in another clue).
'diet' anagrammed gives 'edit'.
'edit' inserted within 'mate' is 'MEDITATE'.
(Other definitions for meditate that I've seen before include "Cogitate" , "Think contemplatively" , "Muse" , "Engage in contemplation" , "Think deeply and quietly" .)