Idler so upset by fighter (7)
I believe the answer is:
soldier
'fighter' is the definition.
(thesaurus)
'idler so upset' is the wordplay.
'upset' indicates anagramming the letters.
'idler'+'so'='idlerso'
'idlerso' anagrammed gives 'SOLDIER'.
'by' acts as a link.
(Other definitions for soldier that I've seen before include "Member of army" , "Person serving in army" , "serviceman" , "Strip of toast to go with a boiled egg" , "Member of the armed forces" .)