English county filing a suit (6)
I believe the answer is:
hearts
'suit' is the definition.
(I've seen this before)
'english county filing a' is the wordplay.
'english county' becomes 'herts' (abbreviation for Hertfordshire).
'filing' indicates putting letters inside (to file something is to sort it into eg a folder).
'herts' placed around 'a' is 'HEARTS'.
(Other definitions for hearts that I've seen before include "Scots club nickname; suit" , "Card game and centres of emotions, it's said" , "quarterdeck?" , "Suit in pack" , "Red playing cards" .)