Old man's taking on fear (5)
I believe the answer is:
dread
'fear' is the definition.
(I know that dread is a type of fear)
'old man's taking on' is the wordplay.
'old man' becomes 'dad' (both can mean one's father).
'taking' indicates putting letters inside.
'on' becomes 're' (regarding).
'dad' going around 're' is 'DREAD'.
(Other definitions for dread that I've seen before include "Show horror" , "Anticipate with horror" , "Deep fear" , "Anticipate fearfully" , "Get cold feet" .)