Once in the middle of kiss, hugged by imbecile (5)
I believe the answer is:
Here is my best explanation:
'once in the middle' is the definition.
(archaic word meaning 'in the middle')
'kiss hugged by imbecile' is the subsidiary indication.
'kiss' becomes 'x'.
'hugged by' is an insertion indicator (I've seen this in other clues).
'imbecile' becomes 'twit'.
'x' inserted into 'twit' is 'twixt'.
'of' is the link.
(Other definitions for twixt that I've seen before include "dividing" and "Between (archaic)".)