Grow less; peter out (7)
I believe the answer is:
dwindle
This clue is a double definition.
'grow less' is the first definition.
(I've seen this before)
'peter out' is the second definition.
(I've seen this before)
(Other definitions for dwindle that I've seen before include "Decline" , "Become steadily less" , "Become gradually less or smaller" , "Grow less - waste away" , "tail off" .)