Supplements daughter found in commercials (4)
I believe the answer is:
adds
'supplements' is the definition.
(I've seen this before)
'daughter found in commercials' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'found in' means one lot of letters goes inside another.
'commercials' becomes 'ads' (ad is short for advert).
'd' inserted into 'ads' is 'ADDS'.
(Other definitions for adds that I've seen before include "Puts in; tacks on" , "Aggregates or appends" , "Appends or increases" , "Counts" , "Finds the sum (4)," .)