Nag about shelter outside in line (7)
I believe the answer is:
tangent
'line' is the definition.
(I know that Tangent is a type of straight line)
'nag about shelter outside' is the wordplay.
'about' indicates an anagram.
'shelter' becomes 'tent' (canvas shelter).
'outside' is an insertion indicator.
'nag' is an anagram of 'ang'.
'ang' inserted within 'tent' is 'TANGENT'.
'in' is the link.
(Other definitions for tangent that I've seen before include "Straight line resting on a circle" , "Line touching curve at one point" , "mathematician's line" , "Particular line" , "Straight line that touches but does not intersect a curve" .)