Thus, a motherless man's a mere child! (3)
I believe the answer is:
son
'child' is the definition.
(male child)
'thus a motherless man's a mere' is the wordplay.
I cannot quite understand how this works, but
'thus' could be 'so' (synonyms) and 'so' is found in the answer.
'man' could be 'n' (knight, a man in chess) and 'n' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for son that I've seen before include "Male descendant" , "Male offspring" , "Native" , "The kind of child" , "Human offspring" .)