Keep in date, cleverly (6)
I believe the answer is:
detain
'keep' is the definition.
(I've seen this before)
'in date cleverly' is the wordplay.
'cleverly' indicates anagramming the letters (a clever or tricky version of the letters).
'in'+'date'='indate'
'indate' is an anagram of 'DETAIN'.
(Other definitions for detain that I've seen before include "Delay or take into custody" , "Hold in detention" , "Hold in custody or delay" , "Keep (someone) held in custody" , "Keep waiting or under restraint" .)