Presumably he can be relied upon for dexterity (5-4,3)
I believe the answer is:
'presumably he can be relied upon for dexterity' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'presumably he' is the wordplay.
I cannot quite see how this works, but
'he' could be 'man' (I've seen this in another clue) and 'man' is found in the answer.
The remaining letters 'righthand' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for right-hand man that I've seen before include "Most trusted assistant" , "Important assistant" , "Indispensable personal aide" , "One's most valuable assistant" , "Indispensable assistant" .)