Identify person keeling over at final whistle (4)
I believe the answer is:
name
'identify' is the definition.
(synonyms)
'person keeling over at final whistle' is the wordplay.
'person' becomes 'man' (man is a kind of person).
'keeling over' says the letters should be written in reverse (in a down clue, words go up).
'at' means one lot of letters go next to another.
'final' says to take the final letters.
The last letter of 'whistle' is 'e'.
'man' in reverse letter order is 'nam'.
'nam'+'e'='NAME'
(Other definitions for name that I've seen before include "Monicker" , "Specify; famous person" , "celebrity" , "Mean sort of appellation" , "By which a person is known" .)