Principal has fallen from swing in corner (5)
I believe the answer is:
angle
'corner' is the definition.
(eg a corner of a shape)
'principal has fallen from swing' is the wordplay.
'principal has fallen from' means to remove the first letter.
'swing' becomes 'dangle' (synonyms).
'dangle' with its first letter removed is 'ANGLE'.
'in' is the link.
(Other definitions for angle that I've seen before include "Standpoint" , "A way of approaching" , "Approach to problem" , "Point of view; old tribesman" , "Early settler" .)