Confirm Valerie has one assignation (8)
I believe the answer is:
validate
'confirm' is the definition.
(I know this)
'valerie has one assignation' is the wordplay.
'valerie' becomes 'val' (I've seen this in other clues).
'has' says to put letters next to each other.
'one' becomes 'I' (Roman numeral).
'assignation' becomes 'date' (I've seen this before).
'val'+'i'+'date'='VALIDATE'
(Other definitions for validate that I've seen before include "Confirm as true" , "Corroborate" , "Prove" , "Long poem" , "Confirm correctness of" .)