Tory heading European research centre? It’s a worry (7)
I believe the answer is:
concern
'a worry' is the definition.
('concern' can be a synonym of 'worry')
'tory heading european research centre?' is the wordplay.
'tory' becomes 'con' (abbreviation for Conservative).
'heading' says to put letters next to each other (some letters go in front of or head others).
'european research centre?' becomes 'cern' (scientific research centre CERN).
'con'+'cern'='CONCERN'
'it's' acts as a link.
(Other definitions for concern that I've seen before include "Have something to do with" , "Affect; solicitude" , "Disquiet" , "Relate to; care" , "Cause of anxiety" .)