Supplement — fuss over date and time (3,2)
I believe the answer is:
add to
'supplement' is the definition.
(I've seen this before)
'fuss over date and time' is the wordplay.
'fuss' becomes 'ado' (synonyms).
'over' is an insertion indicator.
'date' becomes 'd' (common abbreviation eg in DOB, date of birth).
'and' means one lot of letters go next to another.
'time' becomes 't' (abbreviation).
'd'+'t'='dt'
'ado' placed around 'dt' is 'ADD TO'.
(Other definitions for add to that I've seen before include "Increase" , "Supplement" .)