He can handle a horse when there's a wild mare around (7)
I believe the answer is:
manager
'he can handle a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'horse when there's a wild mare around' is the wordplay.
'horse when' becomes 'nag' (nag is a kind of horse. I am not sure about the 'when' bit.).
'there's a wild' indicates anagramming the letters.
'around' indicates putting letters inside.
'mare' is an anagram of 'maer'.
'nag' placed inside 'maer' is 'MANAGER'.
Can you help me to learn more?
(Other definitions for manager that I've seen before include "Controller of resources" , "Supervisor" , "head of department" , "who tries to cope?" , "Executive or controller" .)