Lift, ultimately unreliable, yet to arrive (5)
I believe the answer is:
elate
'lift' is the definition.
(I've seen this before)
'ultimately unreliable yet to arrive' is the wordplay.
'ultimately' says to take the final letters.
'yet to arrive' becomes 'late'.
The final letter of 'unreliable' is 'e'.
'e'+'late'='ELATE'
(Other definitions for elate that I've seen before include "Excite" , "Delight" , "Make joyful and proud" , "Fill with joy" , "Raise the spirits of" .)