Darling behind my back, swoon (4,4)
I believe the answer is:
keel over
'swoon' is the definition.
(thesaurus)
'darling behind my back' is the wordplay.
'darling' becomes 'lover'.
'behind' means one lot of letters go next to another.
'my' becomes 'eek' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'back' says the letters should be written in reverse.
'eek' back-to-front is 'kee'.
'lover' after 'kee' is 'KEEL OVER'.
Can you help me to learn more?
(Other definitions for keel over that I've seen before include "Fall, as in a faint" , "Pass out" , "Suddenly collapse" , "Capsize" .)