The old fellow's heart cry! (4)
I believe the answer is:
yell
'cry' is the definition.
('yell' can be a synonym of 'cry')
'the old fellow's heart' is the wordplay.
'the old' becomes 'ye'.
'heart' indicates the central letters.
The centre of 'fellow' is 'll'.
'ye'+'ll'='YELL'
(Other definitions for yell that I've seen before include "Give a sudden loud cry" , "Shout - Shetland island" , "Bawl, yowl" , "Shetland isle" , "Howl" .)