Small shark's peak energy (4)
I believe the answer is:
tope
'small shark's' is the definition.
(I've seen this in another clue)
'peak energy' is the wordplay.
'peak' becomes 'top' (synonyms).
'energy' becomes 'e' (physics symbol as in E=mco).
'top'+'e'='TOPE'
(Other definitions for tope that I've seen before include "Drink hard regularly" , "What habitual drinkers do" , "Drink a lot like poet" , "Drink to excess" , "Small grey requiem shark" .)