Crossword compiler, about fifty, one taking up residence (7)
I believe the answer is:
settler
'one taking up residence' is the definition.
'crossword compiler about fifty' is the wordplay.
'crossword compiler' becomes 'setter' (I've seen this before).
'about' is an insertion indicator.
'fifty' becomes 'l' (roman numeral).
'setter' enclosing 'l' is 'SETTLER'.
(Other definitions for settler that I've seen before include "newcomer, perhaps" , "New colonist" , "new inhabitant" , "decisive blow" , "who pays his bills" .)