In conversation, shock fast runner (4)
I believe the answer is:
hare
'fast runner' is the definition.
(I've seen this before)
'in conversation shock' is the wordplay.
'in conversation' shows a homophone (sound like).
'shock' becomes 'hair' (I've seen this before).
'hair' sounds like 'HARE'.
(Other definitions for hare that I've seen before include "Sort of rabbit" , "Mammal - run fast" , "Leveret" , "Old saying: 'Let the .... sit" , "'First catch your ....; then cook it'" .)