It's high and to the upper end (3)
I believe the answer is:
tor
'it's high' is the definition.
I can't judge whether this defines the answer.
'to the upper end' is the wordplay.
'end' suggests the final letters.
The last letter of 'upper' is 'r'.
'to'+'r'='TOR'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for tor that I've seen before include "Feature of Dartmoor" , "fell" , "Rot on a Cornish hill" , "Hill or rocky peak" , "Rocky prominence" .)