Directions to measure up commercials in sets of nine (7)
I believe the answer is:
enneads
'sets of nine' is the definition.
(I've seen this in another clue)
'directions to measure up commercials' is the wordplay.
'directions' becomes 'en' (east and north).
'to measure' becomes 'en' (typographical measure).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'commercials' becomes 'ads' (advertisements).
'en' backwards is 'ne'.
'en'+'ne'+'ads'='ENNEADS'
'in' is the link.
(Other definitions for enneads that I've seen before include "Groups appearing one over the eight" , "Eights plus coxes, perhaps" , "groups of nine" , "Groups with members one over the eight" , "Sane end (anag.)" .)