Rule first cells must be set up for admission (6)
I believe the answer is:
avowal
'admission' is the definition.
(I've seen this before)
'rule first cells must be set up' is the wordplay.
'rule' becomes 'law' (law is a kind of rule).
'first cells' becomes 'ova' (more than one ovum or egg cell).
'must be set up' says the letters should be written in reverse (in a down clue, letters go up).
'law'+'ova'='lawova'
'lawova' reversed gives 'AVOWAL'.
'for' acts as a link.
(Other definitions for avowal that I've seen before include "Frank confession" , "Profession of" , "Affirmation" , "testimony?" , "Assertive declaration" .)