Keep date in assembly (6)
I believe the answer is:
detain
'keep' is the definition.
(thesaurus)
'date in assembly' is the wordplay.
'assembly' indicates anagramming the letters.
'date'+'in'='datein'
'datein' anagrammed gives 'DETAIN'.
(Other definitions for detain that I've seen before include "Hold or keep back, maybe for questioning" , "Imprison" , "Keep in official custody" , "Delay, arrest" , "Keep waiting or in confinement" .)