Island’s importance, short of energy (5)
I believe the answer is:
wight
'island's' is the definition.
(Isle of Wight)
'importance short of energy' is the wordplay.
'importance' becomes 'weight' (weight is a kind of importance).
'short of' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'weight' with 'e' removed is 'WIGHT'.
(Other definitions for wight that I've seen before include "Solent isle" , "Courageous < creature" , "South coast island" , "Solent island" , "Sea area in the English Channel" .)