Terminal at Heathrow initially breaks rules (7)
I believe the answer is:
cathode
'terminal' is the definition.
(electrical terminal)
'at heathrow initially breaks rules' is the wordplay.
'initially' indicates taking the first letters.
'breaks' indicates putting letters inside.
'rules' becomes 'code'.
The first letter of 'heathrow' is 'h'.
'at'+'h'='ath'
'ath' inserted into 'code' is 'CATHODE'.
(Other definitions for cathode that I've seen before include "Battery terminal" , "Opposite of anode" , "Positive terminal of a primary cell" , "Electrical terminal" , "Negatively-charged terminal" .)