Bert's friend (5)
I believe the answer is:
ernie
'bert's friend' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ernie that I've seen before include "Premium Bond selecting machine" , "___ Wise, d. 1999" , "Bondman" , "Wise fellow" , "Selector of winning premium bond numbers" .)