Big fuss surrounding limits of diet supplement (3,2)
I believe the answer is:
add to
'supplement' is the definition.
(I've seen this before)
'big fuss surrounding limits of diet' is the wordplay.
'big fuss' becomes 'ado' (I've seen this in another clue).
'surrounding' is an insertion indicator.
'limits of' suggests removing the centre.
'diet' with its middle taken out is 'dt'.
'ado' going around 'dt' is 'ADD TO'.
(Other definitions for add to that I've seen before include "Increase" , "Supplement" .)