Sort problem, having nothing to settle (5)
I believe the answer is:
roost
'settle' is the definition.
(I know this)
'sort problem having nothing' is the wordplay.
'problem' is an anagram indicator.
'having' means one lot of letters goes inside another.
'nothing' becomes 'O' (O resembles 0).
'sort' with letters rearranged gives 'rost'.
'rost' enclosing 'o' is 'ROOST'.
'to' acts as a link.
(Other definitions for roost that I've seen before include "Hen house; perch" , "Shetland tidal water" , "Birds' resting place" , "Temporary place to stay" , "Bird's night-time perch" .)