Charm for a length of time (5)
I believe the answer is:
spell
This clue is a double definition.
'charm' is the first definition.
(both can mean a bit of magic)
'length of time' is the second definition.
(I've seen this in another clue)
'for a' is the link.
(Other definitions for spell that I've seen before include "Deliveries" , "Period (of enchantment?)" , "Period (of work); incantation" , "JP" , "Trance" .)