Snog Kay, young lady Mike dumped (4)
I believe the answer is:
kiss
'snog' is the definition.
(I've seen this before)
'kay young lady mike dumped' is the wordplay.
'kay' becomes 'k'.
'young lady' becomes 'miss' (term of address for a young woman).
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'dumped' indicates named letters should be taken away.
'miss' with 'm' taken out is 'iss'.
'k'+'iss'='KISS'
(Other definitions for kiss that I've seen before include "Transient touch" , "Osculate" , "Touching of lips" , "(Of snooker ball) touch gently; X represents it" , "Act of osculation" .)