Group of students indeed worry, at heart (4)
I believe the answer is:
year
'group of students' is the definition.
(I've seen this before)
'indeed worry at heart' is the wordplay.
'indeed' becomes 'yea' (I've seen this before).
'at heart' indicates the central letters.
The centre of 'worry' is 'r'.
'yea'+'r'='YEAR'
(Other definitions for year that I've seen before include "1961?" , "One intake of undergrads" , "Pupil group" , "Period of earth's orbit" , "Jan to Dec" .)