Shoot up the peak (3)
I believe the answer is:
top
'the peak' is the definition.
('top' can be a synonym of 'peak')
'shoot up' is the wordplay.
'shoot' becomes 'pot' (I've seen this in other clues).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'pot' reversed gives 'TOP'.
(Other definitions for top that I've seen before include "Peak; beat" , "Children's toy" , "cap and > sweater" , "Better" , "Summit or spinning toy" .)