Graduate's returning boy had great strength (6)
I believe the answer is:
samson
'great strength' is the definition.
The definition suggests a singular noun which matches the answer.
'graduate's returning boy' is the wordplay.
'graduate's' becomes 'MA's' (abbreviation for Master of Arts).
'returning' shows that the letters should be reversed in order.
'boy' becomes 'son' (synonyms).
'mas' reversed gives 'sam'.
'sam'+'son'='SAMSON'
'had' is the link.
Can you help me to learn more?
(Other definitions for samson that I've seen before include "Masons (anag.)" , "one with strength in locks" , "Strong Biblical hero" , "Exceptionally strong man" , "Biblical character with supernatural strength" .)