Function a northern group originally held in shelter (7)
I believe the answer is:
tangent
'function' is the definition.
(tangent is a kind of function)
'a northern group originally held in shelter' is the wordplay.
'originally' says to take the initial letters.
'held in' means one lot of letters goes inside another.
'shelter' becomes 'tent' (canvas shelter).
The initial letters of 'northern group' is 'ng'.
'a'+'ng'='ang'
'ang' placed into 'tent' is 'TANGENT'.
(Other definitions for tangent that I've seen before include "mathematician's line" , "Completely different line of thought - trigonometrical function" , "Line touching circumference" , "Geometric line" , "line in mathematics" .)