He has stopped moving around a jumble of letters (7)
I believe the answer is:
settler
'he has stopped moving around a jumble' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'of letters' is the wordplay.
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'letters' with letters rearranged gives 'SETTLER'.
Can you help me to learn more?
(Other definitions for settler that I've seen before include "Early colonist" , "Colonist, pioneer" , "who pays his bills" , "Early mover into an area" , "decisive blow" .)