Fierce swimmer Private Investigator fled — that’s funny! (7)
I believe the answer is:
piranha
'fierce swimmer' is the definition.
(type of fish)
'private investigator fled that's funny' is the wordplay.
'private investigator' becomes 'PI' (abbreviation).
'fled' becomes 'ran' (ran away).
'that's funny' becomes 'ha' (ha is a laugh).
'pi'+'ran'+'ha'='PIRANHA'
(Other definitions for piranha that I've seen before include "Small carnivorous freshwater fish" , "Fierce small fish of the Amazon" , "Voracious eater" , "Ferocious fish" , "Efficient flesh remover" .)