Old man of no fixed address (3)
I believe the answer is:
dad
'old man' is the definition.
(both can mean one's father)
'of no fixed address' is the wordplay.
'of' indicates the answer is hidden within the clue.
'DAD' is hidden within 'no fixed address'.
(Other definitions for dad that I've seen before include "Papa" , "Old man" , "Mum's mate" , "Father, in short" , "Popular and informal father" .)