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' suggests taking the first letters.
'held in' indicates putting letters inside.
'shelter' becomes 'tent' (canvas shelter).
The first letters of 'northern group' is 'ng'.
'a'+'ng'='ang'
'ang' placed inside '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" .)