Regularly prompt old man (3)
I believe the answer is:
pop
'old man' is the definition.
(both can mean one's father)
'regularly prompt' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'prompt' are 'POP'.
(Other definitions for pop that I've seen before include "Youthful music for dad?" , "Sound of a balloon bursting" , "Palindrome of music for the young" , "'American Dad'!" , "Soft drink - father - music - light explosion" .)