He can order a chap to change gear (7)
I believe the answer is:
manager
'he can order a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'chap to change gear' is the wordplay.
'chap' becomes 'man' (I've seen this before**).
'to change' indicates anagramming the letters.
'gear' anagrammed gives 'ager'.
'man'+'ager'='MANAGER'
Can you help me to learn more?
(Other definitions for manager that I've seen before include "executive position" , "Ran game (anag.)" , "Controller of resources" , "Sports team runner" , "Person in charge of staff" .)