Phil bumped into fellow swimmer (7)
I believe the answer is:
dolphin
'swimmer' is the definition.
(dolphin is a kind of fish)
'phil bumped into fellow' is the wordplay.
'bumped' indicates anagramming the letters.
'into' is an insertion indicator.
'fellow' becomes 'don' (I've seen this before).
'phil' with letters rearranged gives 'lphi'.
'lphi' going into 'don' is 'DOLPHIN'.
(Other definitions for dolphin that I've seen before include "Bottlenose creature?" , "member in school" , "Small (bottlenosed) whale" , "Sea mammal like large porpoise, friendly towards people" , "swimmer" .)