You are like Father William, dear fellow! (3,3)
I believe the answer is:
old man
'fellow' is the definition.
(thesaurus)
'you are like father william dear' is the wordplay.
I cannot really see how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is located in the answer.
'like father william' could be 'old' (I've seen this in another clue) and 'old' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for old man that I've seen before include "Informal name for father, husband or ship's captain" , "Husband (inf)" , "Boyfriend" , "Aged male as informal father" , "Husband - father - boss" .)