Niece occasionally engaged in college course (6)
I believe the answer is:
policy
'course' is the definition.
(I have seen 'Course of action' mean 'policy' so perhaps 'course' could also mean 'policy')
'niece occasionally engaged in college' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'engaged in' means one lot of letters goes inside another.
'college' becomes 'poly' (polytechnic).
The alternate letters of 'niece' are 'ic'.
'ic' inserted inside 'poly' is 'POLICY'.
(Other definitions for policy that I've seen before include "Plan of action - insurance document" , "(E.g. government) line" , "Plan of action of organisation - honesty is the best one" , "Certificate of insurance" , "Plan of action adopted by an individual or group" .)