He shows no concern when his right boot needs mending (5)
I believe the answer is:
robot
'he shows no concern when his' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'right boot needs mending' is the wordplay.
I cannot really understand how this works, but
'right' could be 'r' (common abbreviation) and 'r' is found within the answer.
an anagram of 'boot' is 'obot' which is within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for robot that I've seen before include "Automated machine" , "Mechanical worker" , "Lifelike machine" , "Mechanical man" , "Automaton or sci-fi creature" .)