Thus, a motherless man's a mere child! (3)
I believe the answer is:
son
'a mere child' is the definition.
'son' can be an answer for 'child' (son is a kind of child). I'm unsure of the 'a mere' bit.
'thus a motherless man's' is the wordplay.
'thus' becomes 'so' (synonyms).
'a motherless man' becomes 'n' (knight, a man in chess. I am not sure about the 'motherless' bit.).
'so'+'n'='SON'
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" .)