Dangerous swimmer’s power country has cut (7)
I believe the answer is:
piranha
'dangerous swimmer's' is the definition.
(I've seen this in another clue)
'power country has cut' is the wordplay.
'power' becomes 'p' (symbol in physics).
'country' becomes 'Iran' (Iran is a country).
'cut' means to remove the last letter.
'has' with its final letter taken away is 'ha'.
'p'+'iran'+'ha'='PIRANHA'
(Other definitions for piranha that I've seen before include "Efficient flesh remover" , "Fierce small fish of the Amazon" , "Small carnivorous freshwater fish" , "Ferocious fish" , "Voracious eater" .)