Core group of students tackling tricky clue (7)
I believe the answer is:
nucleus
'core group' is the definition.
('nucleus' can be a synonym of 'core group')
'students tackling tricky clue' is the wordplay.
'students' becomes 'nus' (I've seen this in other clues**).
'tackling' indicates putting letters inside (I've seen this in other clues).
'tricky' indicates an anagram.
'clue' is an anagram of 'cleu'.
'nus' going around 'cleu' is 'NUCLEUS'.
'of' is the link.
(Other definitions for nucleus that I've seen before include "part of comet's head" , "Central mass - kernel" , "Core, centre" , "Atomic core" , "Central mass of a cell" .)