Representing Ulster in school cap (6)
I believe the answer is:
finish
'cap' is the definition.
(thesaurus)
'representing ulster in school' is the wordplay.
'representing ulster' becomes 'in' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'school' becomes 'fish'.
'in' inserted into 'fish' is 'FINISH'.
(Other definitions for finish that I've seen before include "Put the final touches to" , "Treatment of surface" , "Conclude, wind up" , "Patina" , "Pefect" .)