Drink advert: Oscar phoned Derek regularly about that (9)
I believe the answer is:
orangeade
'drink' is the definition.
(I've seen this before)
'advert oscar phoned derek regularly about that' is the wordplay.
'advert' becomes 'ad' ('ad' can be a synonym of 'advert').
'oscar' becomes 'o' (phonetic alphabet: alpha, bravo, charlie etc.).
'phoned' becomes 'rang' ('ring' can be a synonym of 'phone').
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'about that' indicates putting letters inside.
The alternate letters of 'derek' are 'ee'.
'o'+'rang'+'ee'='orangee'
'ad' put within 'orangee' is 'ORANGEADE'.
(Other definitions for orangeade that I've seen before include "Bubbly drink" , "Squash" , "Fruit drink" , "Soft citrus drink" , "Pop" .)