Mail ignoring Queen's secret affair (5)
I believe the answer is:
'secret affair' is the definition.
(amour can mean a lover)
'mail ignoring queen's' is the wordplay.
'mail' becomes 'armour' (as in chainmail).
'ignoring' suggests deleting specific letters.
'queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'armour' with 'r' removed is 'AMOUR'.
(Other definitions for amour that I've seen before include "Love affair - loved one" , "Romantic affair" , "20 [AFFAIR]" , "Love-affair" , "French love affair, usually secret" .)