Old man hides concerning phobia (5)
I believe the answer is:
dread
'phobia' is the definition.
(I've seen this in another clue)
'old man hides concerning' is the wordplay.
'old man' becomes 'dad' (both can mean one's father).
'hides' is an insertion indicator.
'concerning' becomes 're' (both can mean on a particular topic).
'dad' going around 're' is 'DREAD'.
(Other definitions for dread that I've seen before include "Fear - dire" , "Awe" , "Be scared of" , "Be very averse to" , "Get cold feet" .)