A no-good shelter outside a line (7)
I believe the answer is:
tangent
'a line' is the definition.
(tangent is a kind of line)
'a no-good shelter outside' is the wordplay.
'no good' becomes 'ng' (abbreviation).
'shelter' becomes 'tent' (canvas shelter).
'outside' is an insertion indicator (some letters go outside others).
'a'+'ng'='ang'
'ang' put into 'tent' is 'TANGENT'.
(Other definitions for tangent that I've seen before include "mostly irrelevant item" , "Completely different line of thought - trigonometrical function" , "A new line of thought" , "Straight line that touches but does not intersect a curve" , "Straight line resting on a circle" .)