He works with heavy materials for Mum and the boy (5)
I believe the answer is:
mason
'he works with heavy materials' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'mum and the boy' is the wordplay.
'mum' becomes 'ma' (synonyms).
'and' says to put letters next to each other.
'the boy' becomes 'son' (synonyms).
'ma'+'son'='MASON'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for mason that I've seen before include "One in lodge" , "one in construction" , "Actor James" , "member of charitable society" , "One works with stone or brick" .)