Shot where force breaches European citadel (6)
I believe the answer is:
effort
'shot' is the definition.
(thesaurus)
'force breaches european citadel' is the wordplay.
'force' becomes 'F' (symbol used in Physics e.g. f=ma).
'breaches' means one lot of letters goes inside another (some letters breach another word).
'european' becomes 'E' (abbreviation e.g. EU).
'citadel' becomes 'fort' (a citadel is a fortified city).
'e'+'fort'='efort'
'f' inserted within 'efort' is 'EFFORT'.
'where' acts as a link.
(Other definitions for effort that I've seen before include "'Endeavour, exertion (6)'" , "force?" , "Try - labour - pains" , "Attempt - hard work" , "Strenuous exertion" .)