Uncle, my boy, is a strong man (6)
I believe the answer is:
samson
'is a strong man' is the definition.
(I've seen this before)
'uncle my boy' is the wordplay.
'uncle' becomes 'sam' (Uncle Sam is an example).
'my boy' becomes 'son' (I've seen this in another clue).
'sam'+'son'='SAMSON'
(Other definitions for samson that I've seen before include "Delilah's muscular boy friend" , "Strong man of the Bible, laid low by Delilah" , "Biblical strongman" , "one with strength in locks" , "Masons (anag.)" .)