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 really see how this works, but
'thus' could be 'so' (synonyms) and 'so' is found within the answer.
'man' could be 'n' (knight, a man in chess) and 'n' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
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" .)