A little gift is opened up in advance (5)
I believe the answer is:
posit
'advance' is the definition.
(both can mean to put forward or suggest)
'a little gift is opened up' is the wordplay.
'a little' says the answer is hidden in the clue (a little bit of the clue).
'up' is a reversal indicator (in down clue: letters go upwards).
'tisop' is hidden within 'gift is opened'.
'tisop' backwards is 'POSIT'.
'in' is the link.
(Other definitions for posit that I've seen before include "forward-looking statement" , "Put forward (an idea)" , "Assume as a fact or principle" , "Put forward (as fact)" , "advance" .)