City defence is breached by East Fife regularly (9)
I believe the answer is:
sheffield
'city' is the definition.
(I've seen this before)
'defence is breached by east fife regularly' is the wordplay.
'defence' becomes 'shield' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'is breached by' indicates putting letters inside (inserted letters break into or breach the word).
'east' becomes 'e' (abbreviation).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'fife' are 'ff'.
'e'+'ff'='eff'
'shield' going around 'eff' is 'SHEFFIELD'.
Can you help me to learn more?
(Other definitions for sheffield that I've seen before include "- - United = the Blades" , "S. Yorkshire city noted for cutlery" , "one of steely reputation" , "City famed for cutlery" , "City known for cutlery" .)