Artist framing outline of outermost hill is Turner (7)
I believe the answer is:
'is turner' is the definition.
(I've seen this in another clue)
'artist framing outline of outermost hill' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'framing' means one lot of letters goes inside another (inserted letters are framed by the outside letters).
'outline of' says to hollow out the word (remove centre letters) (outline refers to the letters on the edges).
'hill' becomes 'tor' (tor is a kind of hill).
'outermost' with its middle removed is 'ot'.
'ra' placed around 'ot' is 'rota'.
(Other definitions for rotator that I've seen before include "Revolver" , "this one is for turning" , "It swivels" , "Something turning" , "anatomical drawing may reveal such muscle" .)